code: def snyder_ac(self, structure):
          
          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.))
docs: Calculates Snyder's acoustic sound velocity (in SI units)
  
          Args:
              structure: pymatgen structure object
  
          Returns: Snyder's acoustic sound velocity (in SI units)