code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          if self.alpha == 0.:
              return -R/(R**2.+z**2./self.q2+self.core2)
          else:
              m2= self.core2+R**2.+z**2./self.q2
              return -m2**(-self.alpha/2.-1.)*R
docs: NAME:
             _Rforce
          PURPOSE:
             evaluate the radial force for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the radial force
          HISTORY:
             2010-07-10 - Written - Bovy (NYU)