code: def _phiforce(self,R,z,phi=0.,t=0.):
          
          return self._scf.phiforce(R,z,phi=phi,use_physical=False)
docs: NAME:
             _phiforce
          PURPOSE:
             evaluate the azimuthal force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the azimuthal force
          HISTORY:
             2016-12-26 - Written - Bovy (UofT)