Spin-free wave functions for small molecules

In an earlier paper, we described a simple version of spin-free calculations of the energy of two three-electron systems, HeH and H₃. Here, we extend our treatment to LiH⁺ and to two four-electron systems, LiH and BeH⁺. The necessary extensions are not trivial, but we are still able to obtain energies in atomic units consistent to at least four figures using very simple approximate wave functions. The wave functions used are given in the text, and are based on two-centre molecular orbitals consisting of exponential functions with varied exponents. Our procedures may be viewed as a convenient fusion of the classic valence-bond and molecular-orbital approaches to molecular structure.