The asymptotic behavior of the dipole and quadrupole moment of a single water molecule from gas phase to large clusters: A QCT analysis

Water undergoes important electrostatic changes within clusters and the liquid phase, in particular the enhancement of the dipole moment of a central water molecule. We systematically investigate the effect that increasing cluster size (up to 21-mer) has on the dipole and quadrupole moments of a water molecule located at the centre of a cluster. Using hundreds of clusters, sampled from a Molecular Dynamics simulation, molecular multipole moments are reconstructed from the atomic multipole moments. These atomic multipole moments are obtained by Quantum Chemical Topology (QCT) from the ab initio electron densities of water clusters, calculated at three levels of theory, viz. B3LYP/6-311+G(2d,p), B3LYP/aug-cc-pVTZ and MP2/6-311+G(2d,p). There is a substantial increase in the average dipole moment with increasing cluster size, with asymptotic behavior emerging.