def _evaluate(self,R,z,phi=0.,t=0.):
        """
        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)
        """
        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