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

        Args:
            structure: pymatgen structure object

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

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.))