Explicitly Correlated Wavefunction for LiH by Perturbation Theory

Start with a local one-electron Hamiltonian and construct a determinantal wavefunction from its eigenfunctions. Taking this as zero-order function, calculate the first-order correction by perturbation theory. This reduces to several two-electron problems, approximate solutions to which may be found variationally, using trial functions involving interelectronic coordinates explicitly with no necessity for evaluation of multielectron integrals. An exception to this is discussed and dealt with. The energy through second order may be calculated as a sum of pair contributions. For LiH at 3.02a0, calculations with screened nucleus and another zero-order potential are performed. It is shown that this energy can be as good as that from the best variational calculations. Problems of convergence are discussed.