Anharmonicity effects on zero point energies of weakly bound molecular clusters

Calculations have been carried out for a series of weakly bound clusters with the evaluation of the harmonic intermolecular vibrational energies and the fully anharmonic zero point energies (ZPE) associated with the weak intermolecular vibrational modes. The potential surfaces of weakly bound complexes often display considerable anharmonicity, but there can be offsetting effects that make for only small differences between harmonic and fully anharmonic ZPE. As well, the existence of nearby multiple minima on the potential surface can lead to sizable differences, and hence, the range of anharmonicity effects in weakly bound clusters is substantial.