code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          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.))
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:
             2013-06-28 - Started - Bovy (IAS)