def _nderiv(self,l,n):
        """
        NAME:
            _nderiv
        PURPOSE:
            evaluate the derivative w.r.t. nu for this potential
        INPUT:
            l - prolate spheroidal coordinate lambda
            n - prolate spheroidal coordinate nu
        OUTPUT:
            derivative w.r.t. nu
        HISTORY:
            2015-02-15 - Written - Trick (MPIA)
        """ 
        return 0.5/nu.sqrt(n)/(nu.sqrt(l)+nu.sqrt(n))**2
NAME:
            _nderiv
        PURPOSE:
            evaluate the derivative w.r.t. nu for this potential
        INPUT:
            l - prolate spheroidal coordinate lambda
            n - prolate spheroidal coordinate nu
        OUTPUT:
            derivative w.r.t. nu
        HISTORY:
            2015-02-15 - Written - Trick (MPIA)