INTERACTIONS ENERGIES ASSOCIATED WITH SHORT INTERMOLECULAR CONTACTS OF C-H BONDS .2. ABINITIO COMPUTATIONAL STUDY OF THE C-H...H-C INTERACTIONS IN METHANE DIMER

On the basis of single-and multireference ab initio calculations on (Be)2 and (H-2)2, we suggest that van der Waals systems with large highest occupied molecular orbital-lowest unoccupied molecular orbital gap can be reasonably well described by single reference MPn (n = 2-4) calculations if the basis set is large enough. The binding energies of C-H...H-C contacts are then examined by performing single reference MPn calculations on (CH4)2 dimer. Our study shows that (CH4)2 is bound in all possible CH4...CH4 arrangements, and that those arrangements with more than one C-H...H-C contact lead to a greater amount of stabilization than does the arrangement with one C-H...H-C contact. The potential energy curves obtained for staggered arrangements of (CH4)2 by the MP2 calculations with the 6-311G(2d,2p) basis set are in close agreement with the experimentally deduced, isotropic potential curve.