Accurate first principles calculation of many-body interactions

The electronic structure Schrödinger equation is solved for the van der Waals complexes spin-polarized H2 and H3, and the closed-shell systems He2 and He3 by Monte Carlo methods. Two types of calculations are performed, variational Monte Carlo, which gives an upper bound to the eigenvalue of the Schrödinger equation, and Green's function Monte Carlo, which can solve the Schrödinger equation exactly within statistical sampling errors. The simulations are carried out on an ETA-10 supercom puter, and already existing computer codes were exten sively modified to ensure highly efficient coding. A major component of the computations was the develop ment of highly optimized many-electron wave functions. The results from the variational Monte Carlo simulations are reported for both the two- and three-body interac tion energies.