A quantum Monte Carlo perturbational study of the He-He interaction

Using a recently proposed quantum Monte Carlo method the exact first-, second-, and third-order Rayleigh-Schrodinger interaction energies of the He-He interaction have been calculated for internuclear distances in the range 1.5- to 7.0 bohr. Putting together these new data with the best ab initio results available, the relative importance of the different contributions appearing in the perturbational expansion of the He-He interaction energy is discussed. In particular, the results show that the third-order Rayleigh-Schrodinger term and the intra-atomic correlation contribution to the second-order component play a significant role. For intermediate and large distances (including the equilibrium distance), it is found that the perturbational expansion limited to the complete first- and second-order, plus the third-order Rayleigh-Schrodinger energy agrees with the best known values of the total interaction energy of the helium dimer. (C) 1996 American Institute of Physics.