Reciprocal space model charge value
        for input squared reciprocal vector.
        Args:
            g2: Square of reciprocal vector

        Returns:
            Charge density at the reciprocal vector magnitude

def rho_rec(self, g2):
        """
        Reciprocal space model charge value
        for input squared reciprocal vector.
        Args:
            g2: Square of reciprocal vector

        Returns:
            Charge density at the reciprocal vector magnitude
        """
        return (self.expnorm / np.sqrt(1 + self.gamma2 * g2) + (
            1 - self.expnorm) * np.exp(-0.25 * self.beta2 * g2))