Nonadiabatic wavefunctions as linear expansions of correlated exponentials. A quantum Monte Carlo application to H-2(+) and Ps(2)

We propose to expand the nonadiabatic solution of the Schrodinger equation as a linear combination of explicitly correlated exponentials. A series of trial wavefunctions has been optimized minimizing the variance of the local energy for the H-2(+) and dipositronium (Ps(2)) molecules in their ground state, without resorting to the Born-Oppenheimer approximation: the calculations have been performed using the variational Monte Carlo method. In a diffusion Monte Carlo simulation a h-term wavefunction allowed us to compute the exact energy of the Ps(2) system -0.51601 hartree with a variance of 0.00001 hartree. (C) 1997 Elsevier Science B.V.