Nonadiabatic wavefunctions as linear expansion of correlated

We propose to expand the nonadiabatic solution of the Schrödinger 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 H2 + and dipositronium (Ps2) molecules in their ground state, without resorting to the Born-Oppenheimer approximation: the calculations have been performed using the Variational Monte Carlo method. A 6-term wavefunction has been used in diffusion Monte Carlo simulations to compute the exact energy of the Ps2 system.