Correlation effects and many-body interactions in water clusters.

Andreas Heßelmann

doi:10.3762/bjoc.14.83

Abstract

Background: The quantum-chemical description of the interactions in water clusters is an essential basis for deriving accurate and physically sound models of the interaction potential for water to be used in molecular simulations. In particular, the role of many-body interactions beyond the two-body interactions, which are often not explicitly taken into account by empirical force fields, can be accurately described by quantum chemistry methods on an adequate level, e.g., random-phase approximation electron correlation methods. The relative magnitudes of the different interaction energy contributions obtained by accurate ab initio calculations can therefore provide useful insights that can be exploited to develop enhanced force field methods.Results: In line with earlier theoretical studies of the interactions in water clusters, it has been found that the main contribution to the many-body interactions in clusters with a size of up to N = 13 molecules are higher-order polarisation interaction terms. Compared to this, many-body dispersion interactions are practically negligible for all studied sytems. The two-body dispersion interaction, however, plays a significant role in the formation of the structures of the water clusters and their stability, since it leads to a distinct compression of the cluster sizes compared to the structures optimized on an uncorrelated level. Overall, the many-body interactions amount to about 13% of the total interaction energy, irrespective of the cluster size. The electron correlation contribution to these, however, amounts to only about 30% to the total many-body interactions for the largest clusters studied and is repulsive for all structures considered in this work.Conclusion: While this shows that three- and higher-body interactions can not be neglected in the description of water complexes, the electron correlation contributions to these are much smaller in comparison to the two-body electron correlation effects. Efficient quantum chemistry approaches for describing intermolecular interactions between water molecules may therefore describe higher-body interactions on an uncorrelated Hartree–Fock level without a serious loss in accuracy.

Highlights

The description of the intermolecular interactions between water molecules is essential for an understanding of the structures and properties of water through the different stages of assemblies, from the dimer over the liquid phase to the bulk phase
While this shows that three- and higher-body interactions can not be neglected in the description of water complexes, the electron correlation contributions to these are much smaller in comparison to the two-body electron correlation effects
Efficient quantum chemistry approaches for describing intermolecular interactions between water molecules may describe higher-body interactions on an uncorrelated Hartree–Fock level without a serious loss in accuracy

Summary

Introduction

The description of the intermolecular interactions between water molecules is essential for an understanding of the structures and properties of water through the different stages of assemblies, from the dimer over the liquid phase to the bulk phase. In order to describe such phenomena, computer simulations have become an indispensable tool, since they enable a description of water on a molecular level that often can provide further insights than are accessible from spectroscopic measurements. The basis for such simulations are the so-called force fields that describe both the covalent as well as the noncovalent interactions within the system. The quantum-chemical description of the interactions in water clusters is an essential basis for deriving accurate and physically sound models of the interaction potential for water to be used in molecular simulations. The relative magnitudes of the different interaction energy contributions obtained by accurate ab initio calculations can provide useful insights that can be exploited to develop enhanced force field methods

Methods

Results

Conclusion