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