Implications of the complete basis set limit in valence bond theory: a case study of molecular hydrogen

The use of larger basis sets to approach the complete basis limit, now common in quantum chemistry, is applied for the first time to a range of valence bond functions for the simplest case of molecular hydrogen. Good convergence of the energy is slow due to difficulty in getting a correct cusp near the nuclei, but it is significant. The form of the orbitals converges much faster, leading to a slight distortion of the valence bond orbitals and an enhanced overlap, irrespective whether the basis set is restricted to basis functions centred on one atom for each valence bond orbital or the full use of the basis set is allowed. This blurs the distinction between these two approaches and shows that basis set restrictions are not tenable in the complete basis set limit. Furthermore, it supports the general use of the full basis as advocated in the spin-coupled and generalised valence bond methods.