The third-order term Fpot(ρ)[ρq] Fpot(ρ)[ρq] above corresponds to the third-order energy in the structural expansion for electron systems. This energy plays an important role in the structural stability of metallic hydrogen ¹⁶ (Z= 1). This is because the kernel (10) has a strong wave-vector dependence, which cannot properly be treated in the square-gradient type theory. This brings a special stability of the structure in which as many equilateral triangles with q=3kF appear in the reciprocal lattice. It requires an anisotropic structure or a periodic modulation of the lattice for an atomic phase of hydrogen. Another possibility is to form a crystal with two ions in a unit cell (molecular phase). In any case, the atomic phase of bcc structure would not be realized just under the melting line as log as the third order term in F_c[ρ_q] does not suppress the effect. We shall investigate this problem numerically in the near future.