Multipole moments of water molecules in clusters and ice Ih from first principles calculations

We have calculated molecular multipole moments for water molecules in clust ers and in ice Ih by partitioning the charge density obtained from first pr inciples calculations. Various schemes for dividing the electronic charge d ensity among the water molecules were used. They include Bader's zero flux surfaces and Voronoi partitioning schemes. A comparison was also made with an induction model including dipole, dipole-quadrupole, quadrupole-quadrupo le polarizability and first hyperpolarizability as well as fixed octopole a nd hexadecapole moments. We have found that the different density partition ing schemes lead to widely different values for the molecular multipoles, i llustrating how poorly defined molecular multipoles are in clusters and con densed environments. For instance, the magnitude of the molecular dipole mo ment in ice Ih ranges between 2.3 D and 3.1 D depending on the partitioning scheme used. Within each scheme, though, the value for the molecular dipol e moment in ice is larger than in the hexamer. The magnitude of the molecul ar dipole moment in the clusters shows a monotonic increase from the gas ph ase value to the one in ice Ih, with the molecular dipole moment in the wat er ring hexamer being smaller than the one in ice Ih for all the partitioni ng schemes used. (C) 1999 American Institute of Physics. [S0021-9606(99)516 37-1].