code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          x,y,z= self._compute_xyz(R,phi,z,t)
          Tp, Tm= self._compute_TpTm(x,y,z)
          return numpy.log((x-self._a+Tm)/(x+self._a+Tp))/2./self._a
docs: NAME:
             _evaluate
          PURPOSE:
             evaluate the potential at R,z
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             Phi(R,z)
          HISTORY:
             2016-11-02 - Started - Bovy (UofT)