ACCURATE ABINITIO POTENTIAL-ENERGY COMPUTATIONS FOR THE H4 SYSTEM - TESTS OF SOME ANALYTIC POTENTIAL-ENERGY SURFACES

The interaction potential energy surface (PES) of H-4 is of great importance for quantum chemistry, as a test case for molecule-molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely, collisional excitation and dissociation of H-2 in molecular clouds, at densities too low to be accessible experimentally. Accurate ab initio energies were computed for 6046 conformations of H-4, using a multiple reference (single and) double excitation configuration interaction (MRD-CI) program. Both systematic and "random" errors were estimated to have an rms size of 0.6 mhartree, for a total rms error of about 0.9 mhartree (or 0.55 kcal/mol) in the final ab initio energy values. It proved possible to include in a self-consistent way ab initio energies calculated by Schwenke, bringing the number of H-4 conformations to 6101. Ab initio energies were also computed for 404 conformations of H-3; adding ab initio energies calculated by other authors yielded a total of 772 conformations of H-3. (The H-3 results, and an improved analytic PES for H-3, are reported elsewhere.) Ab initio energies are tabulated in this paper only for a sample of H-4 conformations; a full list of all 6101 conformations of H-4 (and 772 conformations of H-3) is available from Physics Auxiliary Publication Service (PAPS), or from the authors. The best existing analytic PESs for H-4 are shown to be accurate only for pairs of H-2 molecules with intermolecular separations greater than about 3 bohr (1.6 angstrom). High energy collisions (such as might lead to direct collisional dissociation) cannot be well represented by such surfaces. A more general analytic PES for H-4 is required, which will be accurate for compact (high-energy) conformations and for conformations that cannot be subdivided into a pair of H-2 molecules. Work in progress on devising such a surface (fitted to the 6101 conformations of this work) will be reported in a forthcoming paper.