def _l2deriv(self,l,n):
        """
        NAME:
            _l2deriv
        PURPOSE:
            evaluate the second derivative w.r.t. lambda for this potential
        INPUT:
            l - prolate spheroidal coordinate lambda
            n - prolate spheroidal coordinate nu
        OUTPUT:
            second derivative w.r.t. lambda
        HISTORY:
            2015-02-15 - Written - Trick (MPIA)
        """
        numer = -3.*nu.sqrt(l) - nu.sqrt(n)
        denom = 4. * l**1.5 * (nu.sqrt(l)+nu.sqrt(n))**3
        return numer / denom
NAME:
            _l2deriv
        PURPOSE:
            evaluate the second derivative w.r.t. lambda for this potential
        INPUT:
            l - prolate spheroidal coordinate lambda
            n - prolate spheroidal coordinate nu
        OUTPUT:
            second derivative w.r.t. lambda
        HISTORY:
            2015-02-15 - Written - Trick (MPIA)