Reciprocal space model charge value
        close to reciprocal vector 0 .
        rho_rec(g->0) -> 1 + rho_rec_limit0 * g^2

def rho_rec_limit0(self):
        """
        Reciprocal space model charge value
        close to reciprocal vector 0 .
        rho_rec(g->0) -> 1 + rho_rec_limit0 * g^2
        """
        return -2 * self.gamma2 * self.expnorm - 0.25 * self.beta2 * (
            1 - self.expnorm)