THE RELIABILITY OF THE POINT-CHARGE MODEL REPRESENTING INTERMOLECULAREFFECTS IN AB-INITIO CALCULATIONS

The reliability of modeling ions or polar molecules by point charges i s investigated from the point of view of possible non-physical variati onal distortions of the molecular wavefunction. Mathematical and obser vable-based criteria are given to monitor the extent of variational di stortions. A model system of two H-2 molecules and a cation is studied , allowing investigation of large basis sets. The unphysical distortio n, which tends to be worse as the basis set increases, is already obse rved in split-shell quality bases, if sensitive parameters (e.g. van d er Waals distances) are investigated. It has a negligible influence on less sensitive molecular properties, such as the total electron densi ty, even in large basis sets.