Ar_nH₂O (n=1-14) van der Waals clusters: Size evolution of equilibrium structures

Size evolution of the equilibrium structures of Ar_nH ₂O van der Waals clusters with n=1-14 has been investigated. Pairwise additive jntermolecular potential energy surfaces (IPESs) for Ar ₂H₂O clusters were constructed from the spectroscopically accurate Ar-Ar and anisotropic 3D Ar-H₂O potentials. For each cluster size considered, we determined the global minimum of the respective IPES and several other lowest-lying Ar_nH₂O isomeric structures. This was accomplished by using simulated annealing followed by a direct minimization scheme. The minimum-energy structures of all Ar_nH ₂O clusters considered in this work are fully solvated; up to n=12, the Ar atoms fill a monolayer around H₂O. For n=12, the optimal Ar₁₂H₂O structure has the Ar atoms arranged in a highly symmetrical icosahedron, with H₂O in its center. The icosahedral Ar₁₂H₂O structure is exceptionally stable; the energy gap separating it from the next higher n=12 isomer (289.55 cm^-1) exceeds that for any other cluster in this size range. The observed preference for solvated Ar_nH₂O structures was carefully analyzed in terms of the relative energetic contributions from Ar-Ar and Ar-H₂O interactions. For n≤9, the monolayer, cagelike geometries are favored primarily by providing optimal Ar-H₂O interactions, significantly larger than for alternative Ar_nH₂O structures. For n>9, the solvated Ar_nH₂O isomers offer the best Ar-Ar packing, in addition to the strongest total Ar-H₂O interactions. A detailed comparison was made with the minimum-energy structures of Ar_nHF clusters, determined by us recently [J. Chem. Phys. 100, 7166 (1994)], revealing interesting differences in the growth patterns of the optimal cluster structures.