# Source code for pymatgen.analysis.elasticity.elastic

    # coding: utf-8
    # Copyright (c) Pymatgen Development Team.
    # Distributed under the terms of the MIT License.

    """
    This module provides a class used to describe the elastic tensor,
    including methods used to fit the elastic tensor from linear response
    stress-strain data
    """

    from pymatgen.core.tensors import Tensor, \
        TensorCollection, get_uvec, SquareTensor, DEFAULT_QUAD
    from pymatgen.analysis.elasticity.stress import Stress
    from pymatgen.analysis.elasticity.strain import Strain
    from pymatgen.core.units import Unit
    from scipy.special import factorial
    from scipy.integrate import quad
    from scipy.optimize import root
    from collections import OrderedDict
    from monty.dev import deprecated
    import numpy as np
    import warnings
    import itertools

    import sympy as sp

    __author__ = "Joseph Montoya"
    __copyright__ = "Copyright 2012, The Materials Project"
    __credits__ = ("Maarten de Jong, Ian Winter, Shyam Dwaraknath, "
                   "Mark Asta, Anubhav Jain")
    __version__ = "1.0"
    __maintainer__ = "Joseph Montoya"
    __email__ = "montoyjh@lbl.gov"
    __status__ = "Production"
    __date__ = "July 24, 2018"

    class NthOrderElasticTensor(Tensor):
        """
        An object representing an nth-order tensor expansion
        of the stress-strain constitutive equations
        """
        GPa_to_eV_A3 = Unit("GPa").get_conversion_factor(Unit("eV ang^-3"))
        symbol = "C"

        def __new__(cls, input_array, check_rank=None, tol=1e-4):
            """
            Args:
                input_array ():
                check_rank ():
                tol ():
            """
            obj = super().__new__(
                cls, input_array, check_rank=check_rank)
            if obj.rank % 2 != 0:
                raise ValueError("ElasticTensor must have even rank")
            if not obj.is_voigt_symmetric(tol):
                warnings.warn("Input elastic tensor does not satisfy "
                              "standard voigt symmetries")
            return obj.view(cls)

        @property
        def order(self):
            """
            Order of the elastic tensor
            """
            return self.rank // 2

        def calculate_stress(self, strain):
            """
            Calculate's a given elastic tensor's contribution to the
            stress using Einstein summation

            Args:
                strain (3x3 array-like): matrix corresponding to strain
            """
            strain = np.array(strain)
            if strain.shape == (6,):
                strain = Strain.from_voigt(strain)
            assert strain.shape == (3, 3), "Strain must be 3x3 or voigt-notation"
            stress_matrix = self.einsum_sequence([strain] * (self.order - 1)) / factorial(self.order - 1)
            return Stress(stress_matrix)

        def energy_density(self, strain, convert_GPa_to_eV=True):
            """
            Calculates the elastic energy density due to a strain
            """
            e_density = np.sum(self.calculate_stress(strain) * strain) / self.order
            if convert_GPa_to_eV:
                e_density *= self.GPa_to_eV_A3  # Conversion factor for GPa to eV/A^3
            return e_density

        @classmethod
        def from_diff_fit(cls, strains, stresses, eq_stress=None,
                          order=2, tol=1e-10):
            """

            Args:
                strains ():
                stresses ():
                eq_stress ():
                order ():
                tol ():

            Returns:

            """
            return cls(diff_fit(strains, stresses, eq_stress, order, tol)[order - 2])

    def raise_error_if_unphysical(f):
        """
        Wrapper for functions or properties that should raise an error
        if tensor is unphysical.
        """

        def wrapper(self, *args, **kwargs):
            """
            Args:
                self ():
                *args ():
                **kwargs ():

            Returns:

            """
            if self.k_vrh < 0 or self.g_vrh < 0:
                raise ValueError("Bulk or shear modulus is negative, property "
                                 "cannot be determined")
            return f(self, *args, **kwargs)

        return wrapper

    class ElasticTensor(NthOrderElasticTensor):
        """
        This class extends Tensor to describe the 3x3x3x3
        second-order elastic tensor, C_{ijkl}, with various
        methods for estimating other properties derived from
        the second order elastic tensor
        """

        def __new__(cls, input_array, tol=1e-4):
            """
            Create an ElasticTensor object.  The constructor throws an error if
            the shape of the input_matrix argument is not 3x3x3x3, i. e. in true
            tensor notation.  Issues a warning if the input_matrix argument does
            not satisfy standard symmetries.  Note that the constructor uses
            __new__ rather than __init__ according to the standard method of
            subclassing numpy ndarrays.

            Args:
                input_array (3x3x3x3 array-like): the 3x3x3x3 array-like
                    representing the elastic tensor

                tol (float): tolerance for initial symmetry test of tensor
            """

            obj = super().__new__(cls, input_array,
                                  check_rank=4, tol=tol)
            return obj.view(cls)

        @property
        def compliance_tensor(self):
            """
            returns the Voigt-notation compliance tensor,
            which is the matrix inverse of the
            Voigt-notation elastic tensor
            """
            s_voigt = np.linalg.inv(self.voigt)
            return ComplianceTensor.from_voigt(s_voigt)

        @property
        def k_voigt(self):
            """
            returns the K_v bulk modulus
            """
            return self.voigt[:3, :3].mean()

        @property
        def g_voigt(self):
            """
            returns the G_v shear modulus
            """
            return (2. * self.voigt[:3, :3].trace() -
                    np.triu(self.voigt[:3, :3]).sum() +
                    3 * self.voigt[3:, 3:].trace()) / 15.

        @property
        def k_reuss(self):
            """
            returns the K_r bulk modulus
            """
            return 1. / self.compliance_tensor.voigt[:3, :3].sum()

        @property
        def g_reuss(self):
            """
            returns the G_r shear modulus
            """
            return 15. / (8. * self.compliance_tensor.voigt[:3, :3].trace() -
                          4. * np.triu(self.compliance_tensor.voigt[:3, :3]).sum() +
                          3. * self.compliance_tensor.voigt[3:, 3:].trace())

        @property
        def k_vrh(self):
            """
            returns the K_vrh (Voigt-Reuss-Hill) average bulk modulus
            """
            return 0.5 * (self.k_voigt + self.k_reuss)

        @property
        def g_vrh(self):
            """
            returns the G_vrh (Voigt-Reuss-Hill) average shear modulus
            """
            return 0.5 * (self.g_voigt + self.g_reuss)

        @property
        def y_mod(self):
            """
            Calculates Young's modulus (in SI units) using the
            Voigt-Reuss-Hill averages of bulk and shear moduli
            """
            return 9.e9 * self.k_vrh * self.g_vrh / (3. * self.k_vrh + self.g_vrh)

        def directional_poisson_ratio(self, n, m, tol=1e-8):
            """
            Calculates the poisson ratio for a specific direction
            relative to a second, orthogonal direction

            Args:
                n (3-d vector): principal direction
                m (3-d vector): secondary direction orthogonal to n
                tol (float): tolerance for testing of orthogonality
            """
            n, m = get_uvec(n), get_uvec(m)
            if not np.abs(np.dot(n, m)) < tol:
                raise ValueError("n and m must be orthogonal")
            v = self.compliance_tensor.einsum_sequence([n] * 2 + [m] * 2)
            v *= -1 / self.compliance_tensor.einsum_sequence([n] * 4)
            return v

        def directional_elastic_mod(self, n):
            """
            Calculates directional elastic modulus for a specific vector
            """
            n = get_uvec(n)
            return self.einsum_sequence([n] * 4)

        @raise_error_if_unphysical
        def trans_v(self, structure):
            """
            Calculates transverse sound velocity (in SI units) using the
            Voigt-Reuss-Hill average bulk modulus

            Args:
                structure: pymatgen structure object

            Returns: transverse sound velocity (in SI units)

            """
            nsites = structure.num_sites
            volume = structure.volume
            natoms = structure.composition.num_atoms
            weight = float(structure.composition.weight)
            mass_density = 1.6605e3 * nsites * weight / (natoms * volume)
            if self.g_vrh < 0:
                raise ValueError("k_vrh or g_vrh is negative, "
                                 "sound velocity is undefined")
            return (1e9 * self.g_vrh / mass_density) ** 0.5

        @raise_error_if_unphysical
        def long_v(self, structure):
            """
            Calculates longitudinal sound velocity (in SI units)
            using the Voigt-Reuss-Hill average bulk modulus

            Args:
                structure: pymatgen structure object

            Returns: longitudinal sound velocity (in SI units)

            """
            nsites = structure.num_sites
            volume = structure.volume
            natoms = structure.composition.num_atoms
            weight = float(structure.composition.weight)
            mass_density = 1.6605e3 * nsites * weight / (natoms * volume)
            if self.g_vrh < 0:
                raise ValueError("k_vrh or g_vrh is negative, "
                                 "sound velocity is undefined")
            return (1e9 * (self.k_vrh + 4. / 3. * self.g_vrh) / mass_density) ** 0.5

        @raise_error_if_unphysical
        def snyder_ac(self, structure):
            """
            Calculates Snyder's acoustic sound velocity (in SI units)

            Args:
                structure: pymatgen structure object

            Returns: Snyder's acoustic sound velocity (in SI units)

            """
            nsites = structure.num_sites
            volume = structure.volume
            natoms = structure.composition.num_atoms
            num_density = 1e30 * nsites / volume
            tot_mass = sum([e.atomic_mass for e in structure.species])
            avg_mass = 1.6605e-27 * tot_mass / natoms
            return 0.38483 * avg_mass * ((self.long_v(structure) + 2. * self.trans_v(structure)) / 3.) ** 3. \
                / (300. * num_density ** (-2. / 3.) * nsites ** (1. / 3.))

        @raise_error_if_unphysical
        def snyder_opt(self, structure):
            """
            Calculates Snyder's optical sound velocity (in SI units)

            Args:
                structure: pymatgen structure object

            Returns: Snyder's optical sound velocity (in SI units)

            """
            nsites = structure.num_sites
            volume = structure.volume
            num_density = 1e30 * nsites / volume
            return 1.66914e-23 * (self.long_v(structure) + 2. * self.trans_v(structure)) / 3. \
                / num_density ** (-2. / 3.) * (1 - nsites ** (-1. / 3.))

        @raise_error_if_unphysical
        def snyder_total(self, structure):
            """
            Calculates Snyder's total sound velocity (in SI units)

            Args:
                structure: pymatgen structure object

            Returns: Snyder's total sound velocity (in SI units)

            """
            return self.snyder_ac(structure) + self.snyder_opt(structure)

        @raise_error_if_unphysical
        def clarke_thermalcond(self, structure):
            """
            Calculates Clarke's thermal conductivity (in SI units)

            Args:
                structure: pymatgen structure object

            Returns: Clarke's thermal conductivity (in SI units)

            """
            nsites = structure.num_sites
            volume = structure.volume
            tot_mass = sum([e.atomic_mass for e in structure.species])
            natoms = structure.composition.num_atoms
            weight = float(structure.composition.weight)
            avg_mass = 1.6605e-27 * tot_mass / natoms
            mass_density = 1.6605e3 * nsites * weight / (natoms * volume)
            return 0.87 * 1.3806e-23 * avg_mass ** (-2. / 3.) * mass_density ** (1. / 6.) * self.y_mod ** 0.5

        @raise_error_if_unphysical
        def cahill_thermalcond(self, structure):
            """
            Calculates Cahill's thermal conductivity (in SI units)

            Args:
                structure: pymatgen structure object

            Returns: Cahill's thermal conductivity (in SI units)

            """
            nsites = structure.num_sites
            volume = structure.volume
            num_density = 1e30 * nsites / volume
            return 1.3806e-23 / 2.48 * num_density ** (2. / 3.) * (self.long_v(structure) + 2 * self.trans_v(structure))

        @raise_error_if_unphysical
        def debye_temperature(self, structure):
            """
            Estimates the debye temperature from longitudinal and
            transverse sound velocities

            Args:
                structure: pymatgen structure object

            Returns: debye temperature (in SI units)

            """
            v0 = (structure.volume * 1e-30 / structure.num_sites)
            vl, vt = self.long_v(structure), self.trans_v(structure)
            vm = 3 ** (1. / 3.) * (1 / vl ** 3 + 2 / vt ** 3) ** (-1. / 3.)
            td = 1.05457e-34 / 1.38065e-23 * vm * (6 * np.pi ** 2 / v0) ** (1. / 3.)
            return td

        @deprecated("debye_temperature_from_sound_velocities is now the default"
                    "debye_temperature function, this one will be removed.")
        @raise_error_if_unphysical
        def debye_temperature_from_sound_velocities(self, structure):
            """
            Estimates Debye temperature from sound velocities
            """
            return self.debye_temperature(structure)

        @property
        def universal_anisotropy(self):
            """
            returns the universal anisotropy value
            """
            return 5. * self.g_voigt / self.g_reuss + self.k_voigt / self.k_reuss - 6.

        @property
        def homogeneous_poisson(self):
            """
            returns the homogeneous poisson ratio
            """
            return (1. - 2. / 3. * self.g_vrh / self.k_vrh) / (2. + 2. / 3. * self.g_vrh / self.k_vrh)

        def green_kristoffel(self, u):
            """
            Returns the Green-Kristoffel tensor for a second-order tensor
            """
            return self.einsum_sequence([u, u], "ijkl,i,l")

        @property
        def property_dict(self):
            """
            returns a dictionary of properties derived from the elastic tensor
            """
            props = ["k_voigt", "k_reuss", "k_vrh", "g_voigt", "g_reuss", "g_vrh",
                     "universal_anisotropy", "homogeneous_poisson", "y_mod"]
            return {prop: getattr(self, prop) for prop in props}

        def get_structure_property_dict(self, structure, include_base_props=True,
                                        ignore_errors=False):
            """
            returns a dictionary of properties derived from the elastic tensor
            and an associated structure

            Args:
                structure (Structure): structure object for which to calculate
                    associated properties
                include_base_props (bool): whether to include base properties,
                    like k_vrh, etc.
                ignore_errors (bool): if set to true, will set problem properties
                    that depend on a physical tensor to None, defaults to False
            """
            s_props = ["trans_v", "long_v", "snyder_ac", "snyder_opt",
                       "snyder_total", "clarke_thermalcond", "cahill_thermalcond",
                       "debye_temperature"]
            if ignore_errors and (self.k_vrh < 0 or self.g_vrh < 0):
                sp_dict = {prop: None for prop in s_props}
            else:
                sp_dict = {prop: getattr(self, prop)(structure) for prop in s_props}
            sp_dict["structure"] = structure
            if include_base_props:
                sp_dict.update(self.property_dict)
            return sp_dict

        @classmethod
        def from_pseudoinverse(cls, strains, stresses):
            """
            Class method to fit an elastic tensor from stress/strain
            data.  Method uses Moore-Penrose pseudoinverse to invert
            the s = C*e equation with elastic tensor, stress, and
            strain in voigt notation

            Args:
                stresses (Nx3x3 array-like): list or array of stresses
                strains (Nx3x3 array-like): list or array of strains
            """
            # convert the stress/strain to Nx6 arrays of voigt-notation
            warnings.warn("Pseudoinverse fitting of Strain/Stress lists may yield "
                          "questionable results from vasp data, use with caution.")
            stresses = np.array([Stress(stress).voigt for stress in stresses])
            with warnings.catch_warnings(record=True):
                strains = np.array([Strain(strain).voigt for strain in strains])

            voigt_fit = np.transpose(np.dot(np.linalg.pinv(strains), stresses))
            return cls.from_voigt(voigt_fit)

        @classmethod
        def from_independent_strains(cls, strains, stresses, eq_stress=None,
                                     vasp=False, tol=1e-10):
            """
            Constructs the elastic tensor least-squares fit of independent strains
            Args:
                strains (list of Strains): list of strain objects to fit
                stresses (list of Stresses): list of stress objects to use in fit
                    corresponding to the list of strains
                eq_stress (Stress): equilibrium stress to use in fitting
                vasp (boolean): flag for whether the stress tensor should be
                    converted based on vasp units/convention for stress
                tol (float): tolerance for removing near-zero elements of the
                    resulting tensor
            """
            strain_states = [tuple(ss) for ss in np.eye(6)]
            ss_dict = get_strain_state_dict(strains, stresses, eq_stress=eq_stress)
            if not set(strain_states) <= set(ss_dict.keys()):
                raise ValueError("Missing independent strain states: "
                                 "{}".format(set(strain_states) - set(ss_dict)))
            if len(set(ss_dict.keys()) - set(strain_states)) > 0:
                warnings.warn("Extra strain states in strain-stress pairs "
                              "are neglected in independent strain fitting")
            c_ij = np.zeros((6, 6))
            for i in range(6):
                istrains = ss_dict[strain_states[i]]["strains"]
                istresses = ss_dict[strain_states[i]]["stresses"]
                for j in range(6):
                    c_ij[i, j] = np.polyfit(istrains[:, i], istresses[:, j], 1)[0]
            if vasp:
                c_ij *= -0.1  # Convert units/sign convention of vasp stress tensor
            c = cls.from_voigt(c_ij)
            c = c.zeroed(tol)
            return c

    class ComplianceTensor(Tensor):
        """
        This class represents the compliance tensor, and exists
        primarily to keep the voigt-conversion scheme consistent
        since the compliance tensor has a unique vscale
        """

        def __new__(cls, s_array):
            """
            Args:
                s_array ():
            """
            vscale = np.ones((6, 6))
            vscale[3:] *= 2
            vscale[:, 3:] *= 2
            obj = super().__new__(cls, s_array, vscale=vscale)
            return obj.view(cls)

    class ElasticTensorExpansion(TensorCollection):
        """
        This class is a sequence of elastic tensors corresponding
        to an elastic tensor expansion, which can be used to
        calculate stress and energy density and inherits all
        of the list-based properties of TensorCollection
        (e. g. symmetrization, voigt conversion, etc.)
        """

        def __init__(self, c_list):
            """
            Initialization method for ElasticTensorExpansion

            Args:
                c_list (list or tuple): sequence of Tensor inputs
                    or tensors from which the elastic tensor
                    expansion is constructed.
            """
            c_list = [NthOrderElasticTensor(c, check_rank=4 + i * 2)
                      for i, c in enumerate(c_list)]
            super().__init__(c_list)

        @classmethod
        def from_diff_fit(cls, strains, stresses, eq_stress=None,
                          tol=1e-10, order=3):
            """
            Generates an elastic tensor expansion via the fitting function
            defined below in diff_fit
            """
            c_list = diff_fit(strains, stresses, eq_stress, order, tol)
            return cls(c_list)

        @property
        def order(self):
            """
            Order of the elastic tensor expansion, i. e. the order of the
            highest included set of elastic constants
            """
            return self[-1].order

        def calculate_stress(self, strain):
            """
            Calculate's a given elastic tensor's contribution to the
            stress using Einstein summation
            """
            return sum([c.calculate_stress(strain) for c in self])

        def energy_density(self, strain, convert_GPa_to_eV=True):
            """
            Calculates the elastic energy density due to a strain
            """
            return sum([c.energy_density(strain, convert_GPa_to_eV)
                        for c in self])

        def get_ggt(self, n, u):
            """
            Gets the Generalized Gruneisen tensor for a given
            third-order elastic tensor expansion.

            Args:
                n (3x1 array-like): normal mode direction
                u (3x1 array-like): polarization direction
            """
            gk = self[0].einsum_sequence([n, u, n, u])
            result = -(2 * gk * np.outer(u, u) + self[0].einsum_sequence([n, n])
                       + self[1].einsum_sequence([n, u, n, u])) / (2 * gk)
            return result

        def get_tgt(self, temperature=None, structure=None, quad=None):
            """
            Gets the thermodynamic Gruneisen tensor (TGT) by via an
            integration of the GGT weighted by the directional heat
            capacity.

            See refs:
                R. N. Thurston and K. Brugger, Phys. Rev. 113, A1604 (1964).
                K. Brugger Phys. Rev. 137, A1826 (1965).

            Args:
                temperature (float): Temperature in kelvin, if not specified
                    will return non-cv-normalized value
                structure (float): Structure to be used in directional heat
                    capacity determination, only necessary if temperature
                    is specified
                quad (dict): quadrature for integration, should be
                    dictionary with "points" and "weights" keys defaults
                    to quadpy.sphere.Lebedev(19) as read from file
            """
            if temperature and not structure:
                raise ValueError("If using temperature input, you must also "
                                 "include structure")

            quad = quad if quad else DEFAULT_QUAD
            points = quad['points']
            weights = quad['weights']
            num, denom, c = np.zeros((3, 3)), 0, 1
            for p, w in zip(points, weights):
                gk = ElasticTensor(self[0]).green_kristoffel(p)
                rho_wsquareds, us = np.linalg.eigh(gk)
                us = [u / np.linalg.norm(u) for u in np.transpose(us)]
                for u in us:
                    # TODO: this should be benchmarked
                    if temperature:
                        c = self.get_heat_capacity(temperature, structure, p, u)
                    num += c * self.get_ggt(p, u) * w
                    denom += c * w
            return SquareTensor(num / denom)

        def get_gruneisen_parameter(self, temperature=None, structure=None,
                                    quad=None):
            """
            Gets the single average gruneisen parameter from the TGT.

            Args:
                temperature (float): Temperature in kelvin, if not specified
                    will return non-cv-normalized value
                structure (float): Structure to be used in directional heat
                    capacity determination, only necessary if temperature
                    is specified
                quad (dict): quadrature for integration, should be
                    dictionary with "points" and "weights" keys defaults
                    to quadpy.sphere.Lebedev(19) as read from file
            """
            return np.trace(self.get_tgt(temperature, structure, quad)) / 3.

        def get_heat_capacity(self, temperature, structure, n, u, cutoff=1e2):
            """
            Gets the directional heat capacity for a higher order tensor
            expansion as a function of direction and polarization.

            Args:
                temperature (float): Temperature in kelvin
                structure (float): Structure to be used in directional heat
                    capacity determination
                n (3x1 array-like): direction for Cv determination
                u (3x1 array-like): polarization direction, note that
                    no attempt for verification of eigenvectors is made
                cutoff (float): cutoff for scale of kt / (hbar * omega)
                    if lower than this value, returns 0
            """
            k = 1.38065e-23
            kt = k * temperature
            hbar_w = 1.05457e-34 * self.omega(structure, n, u)
            if hbar_w > kt * cutoff:
                return 0.0
            c = k * (hbar_w / kt) ** 2
            c *= np.exp(hbar_w / kt) / (np.exp(hbar_w / kt) - 1) ** 2
            return c * 6.022e23

        def omega(self, structure, n, u):
            """
            Finds directional frequency contribution to the heat
            capacity from direction and polarization

            Args:
                structure (Structure): Structure to be used in directional heat
                    capacity determination
                n (3x1 array-like): direction for Cv determination
                u (3x1 array-like): polarization direction, note that
                    no attempt for verification of eigenvectors is made
            """
            l0 = np.dot(np.sum(structure.lattice.matrix, axis=0), n)
            l0 *= 1e-10  # in A
            weight = float(structure.composition.weight) * 1.66054e-27  # in kg
            vol = structure.volume * 1e-30  # in m^3
            vel = (1e9 * self[0].einsum_sequence([n, u, n, u])
                   / (weight / vol)) ** 0.5
            return vel / l0

        def thermal_expansion_coeff(self, structure, temperature, mode="debye"):
            """
            Gets thermal expansion coefficient from third-order constants.

            Args:
                temperature (float): Temperature in kelvin, if not specified
                    will return non-cv-normalized value
                structure (Structure): Structure to be used in directional heat
                    capacity determination, only necessary if temperature
                    is specified
                mode (string): mode for finding average heat-capacity,
                    current supported modes are 'debye' and 'dulong-petit'
            """
            soec = ElasticTensor(self[0])
            v0 = (structure.volume * 1e-30 / structure.num_sites)
            if mode == "debye":
                td = soec.debye_temperature(structure)
                t_ratio = temperature / td

                def integrand(x):
                    return (x ** 4 * np.exp(x)) / (np.exp(x) - 1) ** 2
                cv = 9 * 8.314 * t_ratio ** 3 * quad(integrand, 0, t_ratio ** -1)[0]
            elif mode == "dulong-petit":
                cv = 3 * 8.314
            else:
                raise ValueError("Mode must be debye or dulong-petit")
            tgt = self.get_tgt(temperature, structure)
            alpha = np.einsum('ijkl,ij', soec.compliance_tensor, tgt)
            alpha *= cv / (1e9 * v0 * 6.022e23)
            return SquareTensor(alpha)

        def get_compliance_expansion(self):
            """
            Gets a compliance tensor expansion from the elastic
            tensor expansion.
            """
            # TODO: this might have a general form
            if not self.order <= 4:
                raise ValueError("Compliance tensor expansion only "
                                 "supported for fourth-order and lower")
            ce_exp = [ElasticTensor(self[0]).compliance_tensor]
            einstring = "ijpq,pqrsuv,rskl,uvmn->ijklmn"
            ce_exp.append(np.einsum(einstring, -ce_exp[-1], self[1],
                                    ce_exp[-1], ce_exp[-1]))
            if self.order == 4:
                # Four terms in the Fourth-Order compliance tensor
                einstring_1 = "pqab,cdij,efkl,ghmn,abcdefgh"
                tensors_1 = [ce_exp[0]] * 4 + [self[-1]]
                temp = -np.einsum(einstring_1, *tensors_1)
                einstring_2 = "pqab,abcdef,cdijmn,efkl"
                einstring_3 = "pqab,abcdef,efklmn,cdij"
                einstring_4 = "pqab,abcdef,cdijkl,efmn"
                for es in [einstring_2, einstring_3, einstring_4]:
                    temp -= np.einsum(es, ce_exp[0], self[-2], ce_exp[1], ce_exp[0])
                ce_exp.append(temp)
            return TensorCollection(ce_exp)

        def get_strain_from_stress(self, stress):
            """
            Gets the strain from a stress state according
            to the compliance expansion corresponding to the
            tensor expansion.
            """
            compl_exp = self.get_compliance_expansion()
            strain = 0
            for n, compl in enumerate(compl_exp):
                strain += compl.einsum_sequence([stress] * (n + 1)) / factorial(n + 1)
            return strain

        def get_effective_ecs(self, strain, order=2):
            """
            Returns the effective elastic constants
            from the elastic tensor expansion.

            Args:
                strain (Strain or 3x3 array-like): strain condition
                    under which to calculate the effective constants
                order (int): order of the ecs to be returned
            """
            ec_sum = 0
            for n, ecs in enumerate(self[order - 2:]):
                ec_sum += ecs.einsum_sequence([strain] * n) / factorial(n)
            return ec_sum

        def get_wallace_tensor(self, tau):
            """
            Gets the Wallace Tensor for determining yield strength
            criteria.

            Args:
                tau (3x3 array-like): stress at which to evaluate
                    the wallace tensor
            """
            b = 0.5 * (np.einsum("ml,kn->klmn", tau, np.eye(3)) +
                       np.einsum("km,ln->klmn", tau, np.eye(3)) +
                       np.einsum("nl,km->klmn", tau, np.eye(3)) +
                       np.einsum("kn,lm->klmn", tau, np.eye(3)) +
                       -2 * np.einsum("kl,mn->klmn", tau, np.eye(3)))
            strain = self.get_strain_from_stress(tau)
            b += self.get_effective_ecs(strain)
            return b

        def get_symmetric_wallace_tensor(self, tau):
            """
            Gets the symmetrized wallace tensor for determining
            yield strength criteria.

            Args:
                tau (3x3 array-like): stress at which to evaluate
                    the wallace tensor.
            """
            wallace = self.get_wallace_tensor(tau)
            return Tensor(0.5 * (wallace + np.transpose(wallace, [2, 3, 0, 1])))

        def get_stability_criteria(self, s, n):
            """
            Gets the stability criteria from the symmetric
            Wallace tensor from an input vector and stress
            value.

            Args:
                s (float): Stress value at which to evaluate
                    the stability criteria
                n (3x1 array-like): direction of the applied
                    stress
            """
            n = get_uvec(n)
            stress = s * np.outer(n, n)
            sym_wallace = self.get_symmetric_wallace_tensor(stress)
            return np.linalg.det(sym_wallace.voigt)

        def get_yield_stress(self, n):
            """
            Gets the yield stress for a given direction

            Args:
                n (3x1 array-like): direction for which to find the
                    yield stress
            """
            # TODO: root finding could be more robust
            comp = root(self.get_stability_criteria, -1, args=n)
            tens = root(self.get_stability_criteria, 1, args=n)
            return (comp.x, tens.x)

    # TODO: abstract this for other tensor fitting procedures
    def diff_fit(strains, stresses, eq_stress=None, order=2, tol=1e-10):
        """
        nth order elastic constant fitting function based on
        central-difference derivatives with respect to distinct
        strain states.  The algorithm is summarized as follows:

        1. Identify distinct strain states as sets of indices
           for which nonzero strain values exist, typically
           [(0), (1), (2), (3), (4), (5), (0, 1) etc.]
        2. For each strain state, find and sort strains and
           stresses by strain value.
        3. Find first, second .. nth derivatives of each stress
           with respect to scalar variable corresponding to
           the smallest perturbation in the strain.
        4. Use the pseudoinverse of a matrix-vector expression
           corresponding to the parameterized stress-strain
           relationship and multiply that matrix by the respective
           calculated first or second derivatives from the
           previous step.
        5. Place the calculated nth-order elastic
           constants appropriately.

        Args:
            order (int): order of the elastic tensor set to return
            strains (nx3x3 array-like): Array of 3x3 strains
                to use in fitting of ECs
            stresses (nx3x3 array-like): Array of 3x3 stresses
                to use in fitting ECs.  These should be PK2 stresses.
            eq_stress (3x3 array-like): stress corresponding to
                equilibrium strain (i. e. "0" strain state).
                If not specified, function will try to find
                the state in the list of provided stresses
                and strains.  If not found, defaults to 0.
            tol (float): value for which strains below
                are ignored in identifying strain states.

        Returns:
            Set of tensors corresponding to nth order expansion of
            the stress/strain relation
        """
        strain_state_dict = get_strain_state_dict(
            strains, stresses, eq_stress=eq_stress, tol=tol,
            add_eq=True, sort=True)

        # Collect derivative data
        c_list = []
        dei_dsi = np.zeros((order - 1, 6, len(strain_state_dict)))
        for n, (strain_state, data) in enumerate(strain_state_dict.items()):
            hvec = data["strains"][:, strain_state.index(1)]
            for i in range(1, order):
                coef = get_diff_coeff(hvec, i)
                dei_dsi[i - 1, :, n] = np.dot(coef, data["stresses"])

        m, absent = generate_pseudo(list(strain_state_dict.keys()), order)
        for i in range(1, order):
            cvec, carr = get_symbol_list(i + 1)
            svec = np.ravel(dei_dsi[i - 1].T)
            cmap = dict(zip(cvec, np.dot(m[i - 1], svec)))
            c_list.append(v_subs(carr, cmap))
        return [Tensor.from_voigt(c) for c in c_list]

    def find_eq_stress(strains, stresses, tol=1e-10):
        """
        Finds stress corresponding to zero strain state in stress-strain list

        Args:
            strains (Nx3x3 array-like): array corresponding to strains
            stresses (Nx3x3 array-like): array corresponding to stresses
            tol (float): tolerance to find zero strain state
        """
        stress_array = np.array(stresses)
        strain_array = np.array(strains)
        eq_stress = stress_array[np.all(abs(strain_array) < tol, axis=(1, 2))]

        if eq_stress.size != 0:
            all_same = (abs(eq_stress - eq_stress[0]) < 1e-8).all()
            if len(eq_stress) > 1 and not all_same:
                raise ValueError("Multiple stresses found for equilibrium strain"
                                 " state, please specify equilibrium stress or  "
                                 " remove extraneous stresses.")
            eq_stress = eq_stress[0]
        else:
            warnings.warn("No eq state found, returning zero voigt stress")
            eq_stress = Stress(np.zeros((3, 3)))
        return eq_stress

    def get_strain_state_dict(strains, stresses, eq_stress=None,
                              tol=1e-10, add_eq=True, sort=True):
        """
        Creates a dictionary of voigt-notation stress-strain sets
        keyed by "strain state", i. e. a tuple corresponding to
        the non-zero entries in ratios to the lowest nonzero value,
        e.g. [0, 0.1, 0, 0.2, 0, 0] -> (0,1,0,2,0,0)
        This allows strains to be collected in stencils as to
        evaluate parameterized finite difference derivatives

        Args:
            strains (Nx3x3 array-like): strain matrices
            stresses (Nx3x3 array-like): stress matrices
            eq_stress (Nx3x3 array-like): equilibrium stress
            tol (float): tolerance for sorting strain states
            add_eq (bool): flag for whether to add eq_strain
                to stress-strain sets for each strain state
            sort (bool): flag for whether to sort strain states

        Returns:
            OrderedDict with strain state keys and dictionaries
            with stress-strain data corresponding to strain state
        """
        # Recast stress/strains
        vstrains = np.array([Strain(s).zeroed(tol).voigt for s in strains])
        vstresses = np.array([Stress(s).zeroed(tol).voigt for s in stresses])
        # Collect independent strain states:
        independent = set([tuple(np.nonzero(vstrain)[0].tolist())
                           for vstrain in vstrains])
        strain_state_dict = OrderedDict()
        if add_eq:
            if eq_stress is not None:
                veq_stress = Stress(eq_stress).voigt
            else:
                veq_stress = find_eq_stress(strains, stresses).voigt

        for n, ind in enumerate(independent):
            # match strains with templates
            template = np.zeros(6, dtype=bool)
            np.put(template, ind, True)
            template = np.tile(template, [vstresses.shape[0], 1])
            mode = (template == (np.abs(vstrains) > 1e-10)).all(axis=1)
            mstresses = vstresses[mode]
            mstrains = vstrains[mode]
            # Get "strain state", i.e. ratio of each value to minimum strain
            min_nonzero_ind = np.argmin(np.abs(np.take(mstrains[-1], ind)))
            min_nonzero_val = np.take(mstrains[-1], ind)[min_nonzero_ind]
            strain_state = mstrains[-1] / min_nonzero_val
            strain_state = tuple(strain_state)

            if add_eq:
                # add zero strain state
                mstrains = np.vstack([mstrains, np.zeros(6)])
                mstresses = np.vstack([mstresses, veq_stress])
            # sort strains/stresses by strain values
            if sort:
                mstresses = mstresses[mstrains[:, ind[0]].argsort()]
                mstrains = mstrains[mstrains[:, ind[0]].argsort()]
            strain_state_dict[strain_state] = {"strains": mstrains,
                                               "stresses": mstresses}
        return strain_state_dict

    def generate_pseudo(strain_states, order=3):
        """
        Generates the pseudoinverse for a given set of strains.

        Args:
            strain_states (6xN array like): a list of voigt-notation
                "strain-states", i. e. perturbed indices of the strain
                as a function of the smallest strain e. g. (0, 1, 0, 0, 1, 0)
            order (int): order of pseudoinverse to calculate

        Returns:
            mis: pseudo inverses for each order tensor, these can
                be multiplied by the central difference derivative
                of the stress with respect to the strain state
            absent_syms: symbols of the tensor absent from the PI
                expression
        """
        s = sp.Symbol('s')
        nstates = len(strain_states)
        ni = np.array(strain_states) * s
        mis, absent_syms = [], []
        for degree in range(2, order + 1):
            cvec, carr = get_symbol_list(degree)
            sarr = np.zeros((nstates, 6), dtype=object)
            for n, strain_v in enumerate(ni):
                # Get expressions
                exps = carr.copy()
                for i in range(degree - 1):
                    exps = np.dot(exps, strain_v)
                exps /= np.math.factorial(degree - 1)
                sarr[n] = [sp.diff(exp, s, degree - 1) for exp in exps]
            svec = sarr.ravel()
            present_syms = set.union(*[exp.atoms(sp.Symbol) for exp in svec])
            absent_syms += [set(cvec) - present_syms]
            m = np.zeros((6 * nstates, len(cvec)))
            for n, c in enumerate(cvec):
                m[:, n] = v_diff(svec, c)
            mis.append(np.linalg.pinv(m))
        return mis, absent_syms

    def get_symbol_list(rank, dim=6):
        """
        Returns a symbolic representation of the voigt-notation
        tensor that places identical symbols for entries related
        by index transposition, i. e. C_1121 = C_1211 etc.

        Args:
            dim (int): dimension of matrix/tensor, e. g. 6 for
                voigt notation and 3 for standard
            rank (int): rank of tensor, e. g. 3 for third-order ECs

        Returns:
            c_vec (array): array representing distinct indices
            c_arr (array): array representing tensor with equivalent
                indices assigned as above
        """
        indices = list(
            itertools.combinations_with_replacement(range(dim), r=rank))
        c_vec = np.zeros(len(indices), dtype=object)
        c_arr = np.zeros([dim] * rank, dtype=object)
        for n, idx in enumerate(indices):
            c_vec[n] = sp.Symbol('c_' + ''.join([str(i) for i in idx]))
            for perm in itertools.permutations(idx):
                c_arr[perm] = c_vec[n]
        return c_vec, c_arr

    def subs(entry, cmap):
        """
        Sympy substitution function, primarily for the purposes
        of numpy vectorization

        Args:
            entry (symbol or exp): sympy expr to undergo subs
            cmap (dict): map for symbols to values to use in subs

        Returns:
            Evaluated expression with substitution
        """
        return entry.subs(cmap)

    # Vectorized functions
    v_subs = np.vectorize(subs)
    v_diff = np.vectorize(sp.diff)

    def get_diff_coeff(hvec, n=1):
        """
        Helper function to find difference coefficients of an
        derivative on an arbitrary mesh.

        Args:
            hvec (1D array-like): sampling stencil
            n (int): degree of derivative to find
        """
        hvec = np.array(hvec, dtype=np.float)
        acc = len(hvec)
        exp = np.column_stack([np.arange(acc)] * acc)
        a = np.vstack([hvec] * acc) ** exp
        b = np.zeros(acc)
        b[n] = factorial(n)
        return np.linalg.solve(a, b)