def expQsdsds(self, s):
        '''
        Returns
        -------
        Qtdtdt :  Returns V_{ij} \lambda_j^2 e^{\lambda_j s**2} V^{-1}_{jk}
                This is the second derivative of the branch probability wrt time
        '''
        t=s**2
        elt = self._exp_lt(t)
        lambda_eLambdaT = np.diag(elt*(4.0*t*self.eigenvals**2 + 2.0*self.eigenvals))
        Qsdsds = self.v.dot(lambda_eLambdaT.dot(self.v_inv))
        return Qsdsds