code: def sigma2Derivative(self,R,log=False):
          
          if log:
              return -2./self._params[1]
          else:
              return self._params[2]**2.*sc.exp(-2.*(R-1.)/self._params[1])\
                  *(-2./self._params[1])
docs: NAME:
             sigmaDerivative
          PURPOSE:
             return the derivative wrt R of the sigma_R^2 profile at this R
          INPUT:
             R - Galactocentric radius (/ro)
             log - if True, return the derivative of the log (default: False)
          OUTPUT:
             Sigma_R^2'(R) or (log Sigma_R^2(r) )'
          HISTORY:
             2011-03-24 - Written - Bovy (NYU)