Abstract: The properties of valence shell excitation n-electron wavefunctions with symplectic symme-tries have been discussed. This kind of wave-functions is suitable for both closed-shell and open-shell systems. As illustrations, the ground states of Li₂, LiH₂^- and CH₃⁺ are calculated. The calculated results show that this kind of wavefunctions is capable of describing electron correlations, and the orbitals obtained from the calculation are all natural orbitals.

Key words: Symplectic symmetry, Valence shell excitation, Natural orbital, Electron correlation