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:
           2013-06-28 - Started - Bovy (IAS)
        """
        r= nu.sqrt(R**2.+z**2.)
        return 2.*nu.pi*self.rc**(3.-self.alpha)/r*(r/self.rc*special.gamma(1.-self.alpha/2.)*special.gammainc(1.-self.alpha/2.,(r/self.rc)**2.)-special.gamma(1.5-self.alpha/2.)*special.gammainc(1.5-self.alpha/2.,(r/self.rc)**2.))
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:
           2013-06-28 - Started - Bovy (IAS)