  The interaction potential energy surface (PES) of He-H2 is of great
importance for quantum chemistry, as the simplest test case for interactions
between a molecule and a closed-shell atom. It is also required for a detailed
understanding of certain astrophysical processes, namely collisional excitation
and dissociation of H2 in molecular clouds, at densities too low to be
accessible experimentally. A new set of 23703 ab initio energies was computed,
for He-H2 geometries where the interaction energy was expected to be
non-negligible. These have an estimated rms "random" error of about 0.2
millihartree and a systematic error of about 0.6 millihartree (0.4 kcal/mol). A
new analytic He-H2 PES, with 112 parameters, was fitted to 20203 of these new
ab initio energies (and to an additional 4862 points generated at large
separations). This yielded an improvement by better than an order of magnitude
in the fit to the interaction region, relative to the best previous surfaces
(which were accurate only for near-equilibrium H2 molecule sizes). This new PES
has an rms error of 0.95 millihartree (0.60 kcal/mole) relative to the the
14585 ab initio energies that lie below twice the H2 dissociation energy, and
2.97 millihartree (1.87 kcal/mole) relative to the full set of 20203 ab initio
energies (the fitting procedure used a reduced weight for high energies,
yielding a weighted rms error of 1.42 millihartree, i.e., 0.89 kcal/mole).
These rms errors are comparable to the estimated error in the ab initio
energies themselves; the conical intersection between the ground state and the
first excited state is the largest source of error in the PES.
