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)

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