 
Summary: Exact quantum Monte Carlo calculations of the potential energy surface
for the reaction H+H,+H,+H
Drake L. Diedrich and James B. Anderson
Department of Chemistry,ThePennsylvaniaState University, University Park, Pennsylvania16802
(Received 13 December 1993; accepted 15 February 1994)
We report "exact" quantum Monte Carlo calculations of the potential energy surface for the reaction
H+H2+H2+H. The method used is free of systematic error. The statistical or sampling error was
reduced to 20.10 kcaYmo1 for severaI hundred points distributed across the surface, to 20.02
kcal/mol for the minimum energy approach of H to HZ, to kO.02 kcal/mol near the saddle point, and
to +O.Ol kcal/mol at the saddle point. The upper and lower surfaces in the region of the JahnTeller
cusp were determined with a statistical error of r0.2 kcal/mol.
INTRODUCTION
In a previous paper' we reported quantum Monte Carlo
calculations of the barrier. height for the reaction
H+H,+H,+H. The energy was determined to be 9.61r0.01
kcal/mol at the saddle point. In this paper we report addi
tional calculations to accuracies of 20.1 kca.l/mol or better
for points distributed over the complete potential energy sur
face.
The first ab initio &lculations of the HHH surface
