def surfacemassDerivative(self,R,log=False):
        """
        NAME:
           surfacemassDerivative
        PURPOSE:
           return the derivative wrt R of the surface density profile at this R
        INPUT:
           R - Galactocentric radius (/ro)
           log - if True, return the derivative of the log (default: False)
        OUTPUT:
           Sigma'(R) or (log Sigma(r) )'
        HISTORY:
           2010-03-26 - Written - Bovy (NYU)
        """
        if log:
            return -1./self._params[0]
        else:
            return -sc.exp(-R/self._params[0])/self._params[0]