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

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