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

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