Calculates Snyder's acoustic sound velocity (in SI units)

        Args:
            structure: pymatgen structure object

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

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.))