def _lnderiv(self,l,n):
        """
        NAME:
            _lnderiv
        PURPOSE:
            evaluate the mixed derivative w.r.t. lambda and nu for this potential
        INPUT:
            l - prolate spheroidal coordinate lambda
            n - prolate spheroidal coordinate nu
        OUTPUT:
            d2phi/dl/dn
        HISTORY:
            2015-02-13 - Written - Trick (MPIA)
        """
        return -0.5/(nu.sqrt(l) * nu.sqrt(n) * (nu.sqrt(l)+nu.sqrt(n))**3)