AN EXACT QUANTUM MONTE-CARLO CALCULATION OF THE HELIUM HELIUM INTERMOLECULAR POTENTIAL

We report ''exact'' ab initio calculations of potential energies for t he interaction of two helium atoms. The quantum Monte Carlo method use d is exact in that it requires no mathematical or physical approximati ons beyond those of the Schrodinger equation. As in most Monte Carlo m ethods there is a statistical or sampling error which is readily estim ated. For the equilibrium internuclear distance of 5.6 bohr, the calcu lated electronic energy is - 5.807 483 6 +/- 0.000 000 3 hartrees and the corresponding well depth (epsilon/k) is 11.01 +/- 0.10 K. The calc ulated total energies are approximately 0.004 hartrees or 1200 K below the most recent variational calculations of Liu and McLean [J. Chem. Phys. 92, 2348 (1989)]. The calculated interaction energies are in exc ellent agreement with the interaction energies of Liu and McLean and w ith a recent experimental/theoretical compromise potential energy curv e of Aziz and Slaman [J. Chem. Phys. 94, 8047 (199 1 ) ] which success fully predicts a variety of experimental measurements. The error bars of the ''exact'' quantum Monte Carlo interaction energies straddle the Liu-McLean and Aziz-Slaman results. The Monte Carlo results support t he existence of a bound dimer state.