docs: 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)