AB-INITIO CALCULATIONS ON THE USE OF HELIUM AND NEON AS PROBES OF THEVAN-DER-WAALS SURFACES OF MOLECULES

Interaction energies and geometries for van der Waals (vdW) complexes from ab initio calculations must take into account limitations associa ted with basis set flexibility, electron correlation, and basis set su perposition error (BSSE). Presented are a variety of nb initio calcula tions on the helium, neon, and methane homodimers and the helium-metha ne and neon-methane dimers. Systematic changes in polarization and dif fuse functions, the treatment of electron correlation, and the influen ce of BSSE correction were undertaken as a function of interaction dis tance. Calculations on the helium and neon homodimers were compared wi th available empirical data. Analysis of the contribution of the BSSE correction using the function counterpoise method indicates increases in minimum energy interaction distance when BSSE corrections are inclu ded. The presence of diffuse functions minimize the magnitude of the B SSE. The present calculations lead to the selection of the MP3/6-311+G(3d,3p) without BSSE correction as an appropriate level of theory for the calculation of relative interaction energies and geometries for d ifferent orientations between either helium or neon and molecules. An exponent for an sp diffuse function on helium was determined as part o f the present study.