ANTISYMMETRY IN THE QUANTUM MONTE-CARLO METHOD WITH THE A-FUNCTION TECHNIQUE - H-2 B(3)SIGMA(U)(+), H-2 C(3)PI(U), HE 1 S-3

The diffusion Monte Carlo method for the solution of the Schrodinger equation for atomic and molecular systems is extended to incorporate the antisymmetry constraint. The sign problem is treated constraining the diffusion process of signed walkers within the fluctuating nodes of a function A, defined as sum of Gaussians centered on the psips. The function A is able to build up the full nodal surface in the 3N dimensional space. The algorithm is shown to scale as N(alpha)3 + N(beta)3 where N(alpha) and N(beta) are the number of alpha and beta electrons. Results are given for the b 3SIGMA(u)+ and c 3PI(u) states of H-2 and the 1 3S state of He.