code: def _R2deriv(self,R,z,phi=0.,t=0.):
          
          r= numpy.sqrt(R**2.+z**2.)
          x= r/self.a
          return -numpy.pi/x**3./r**2.*(-4.*R**2.*r**3./(self.a**2.+r**2.)/(self.a+r)+(z**2.-2.*R**2.)*(numpy.pi-2.*numpy.arctan(1./x)-2.*numpy.log(1.+x)-numpy.log(1.+x**2.)))
docs: NAME:
             _Rderiv
          PURPOSE:
             evaluate the second radial derivative for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the second radial derivative
          HISTORY:
             2013-04-10 - Written - Bovy (IAS)