VALIDITY OF THE FUNCTION COUNTERPOISE METHOD AND ABINITIO CALCULATIONS OF VANDERWAALS INTERACTION ENERGY

A scheme to test the validity of the function counterpoise correction method for the basis set superposition error has been designed and presented with ab initio calculations of the Ne-Ne interaction energy at the distance of 4.0 angstrom using a series of extended basis sets. The efficiency of the extended basis functions and the convergence of the calculated interaction energy with respect to basis set and to the Moller-Plesset approximation level have also been studied. The calculations and careful analysis have confirmed the validity of the function counterpoise method; namely, it produces negligible overcorrections for the basis set superposition error compared to other errors caused by the deficiency of basis set and calculation method. The diffuse functions (including the diffuse polarization functions) have been found to be more effective than the regular polarization functions in the description of the interaction energy. The higher Moller-Plesset approximation levels (MP3 or MP4) have been determined to improve the calculated interaction energy significantly over lower levels of the method (MP2 or MP3). This is only when the basis set in calculations exceeds a certain quality level. The higher Moller-Plesset levels are also subject to relatively larger overcorrections for the basis set superposition error.