Effect of basis set on the calculated geometry of the CH4[middot] HCl complex

Abstract

The CH₄ · HCl complex is studied using a variety of basis sets at the MP2 level of theory. It is found that the calculated geometry depends critically on the precise nature of the basis set used, with a particularly important rôle played by polarization and diffuse functions. It is established, that the ‘best’ basis sets lead to a C_3v geometry, in agreement with the conclusions reached by microwave spectroscopy. The geometry is then recalculated at the MP4(SDQ) and QCISD level of theory, and the binding energy of the complex is calculated at the CCSD(T) level of theory.