Highly Compact Wavefunctions for Four‐Body Systems

Wavefunctions which are compact, but still quite accurate, are extremely valuable as tools for gaining understanding of quantum systems. This presentation reviews methods applicable to systems consisting of four or fewer particles, with an emphasis on expansions in basis functions that depend exponentially on all the interparticle distances. Applications to a variety of three and four‐body systems are reviewed, and the virtues of these methods are illustrated, first by detailed studies we have carried out on three‐body systems (the He isoelectronic series), and second, by work now in progress on a four‐body system (the Li atom). It is shown (in computations that are still ongoing) that an expansion with as few as six exponential basis functions is capable of reproducing the nonrelativistic Li ground‐state energy to within about 230 microhartrees of the exact value, an error far less than has been obtained for previously reported basis‐set expansions of comparable length.