code: def r2deriv(self,R,z,phi=0.,t=0.):
          
         
          r= nu.sqrt(R**2.+z**2.)       
          return (self.R2deriv(R,z,phi=phi,t=t,use_physical=False)*R/r\
              +self.Rzderiv(R,z,phi=phi,t=t,use_physical=False)*z/r)*R/r\
              +(self.Rzderiv(R,z,phi=phi,t=t,use_physical=False)*R/r\
              +self.z2deriv(R,z,phi=phi,t=t,use_physical=False)*z/r)*z/r
docs: NAME:
  
             r2deriv
  
          PURPOSE:
  
             evaluate the second spherical radial derivative
  
          INPUT:
  
             R - Cylindrical Galactocentric radius (can be Quantity)
  
             z - vertical height (can be Quantity)
  
             phi - azimuth (optional; can be Quantity)
  
             t - time (optional; can be Quantity)
  
          OUTPUT:
  
             d2phi/dr2
  
          HISTORY:
  
             2018-03-21 - Written - Webb (UofT)