code: def asymmetricdrift(self,R):
          
          sigmaR2= self.targetSigma2(R,use_physical=False)
          return sigmaR2/2./R**self._beta*(1./self._gamma**2.-1.
                                           -R*self._surfaceSigmaProfile.surfacemassDerivative(R,log=True)
                                           -R*self._surfaceSigmaProfile.sigma2Derivative(R,log=True))
docs: NAME:
  
             asymmetricdrift
  
          PURPOSE:
  
             estimate the asymmetric drift (vc-mean-vphi) from an approximation to the Jeans equation
  
          INPUT:
  
             R - radius at which to calculate the asymmetric drift (can be Quantity)
  
          OUTPUT:
  
             asymmetric drift at R
  
          HISTORY:
  
             2011-04-02 - Written - Bovy (NYU)