Accurate SCF calculations have been carried out to investigate the potential of interaction for pairs and triplets of water molecules. The most stable pair configuration involves a linear hydrogen bond of length ROO = 3.00 Å and strength 4.72 kcal/mole. Three-molecule nonadditivities are large in magnitude and vary in sign according to the hydrogen-bond pattern involved. In both aqueous liquids and solids, the net trimer nonadditivity effect amounts to increased binding energy, decreased neighbor distance, and slightly enhanced tendency toward perfect tetrahedral coordination symmetry. The nonadditivity furthermore is inconsistent with the phenomenology of simple mutual electrostatic polarization between neighboring molecules.

Articles you may be interested inCorrelation consistent basis sets for molecular core-valence effects with explicitly correlated wave functions: The atoms B-Ne and Al-Ar Accuracy of electronic wave functions in quantum Monte Carlo: The effect of high-order correlationsWe apply the variational Monte Carlo method to the atoms He through Ne. Our trial wave function is of the form introduced by Boys and Handy. We use the Monte Carlo method to calculate the first and second derivatives of an unreweighted variance and apply Newton's method to minimize this variance. We motivate the form ofthe correlation function using the local current conservation arguments of Feynman and Cohen. Using a self-consistent field wave function multiplied by a Boys and Handy correlation function, we recover a large fraction ofthe correlation energy of these atoms. We give the value of all variational parameters necessary to reproduce our wave functions. The method can be extended easily to other atoms and to molecules. 4172

We report quantum five-dimensional (5D) calculations of the energy levels and wave functions of the hydrogen molecule, para-H2 and ortho-H2, confined inside the small dodecahedral (H2O)20 cage of the sII clathrate hydrate. All three translational and the two rotational degrees of freedom of H2 are included explicitly, as fully coupled, while the cage is treated as rigid. The 5D potential energy surface (PES) of the H2-cage system is pairwise additive, based on the high-quality ab initio 5D (rigid monomer) PES for the H2-H2O complex. The bound state calculations involve no dynamical approximations and provide an accurate picture of the quantum 5D translation-rotation dynamics of H2 inside the cage. The energy levels are assigned with translational (Cartesian) and rotational quantum numbers, based on calculated root-mean-square displacements and probability density plots. The translational modes exhibit negative anharmonicity. It is found that j is a good rotational quantum number, while the threefold degeneracy of the j = 1 level is lifted completely. There is considerable translation-rotation coupling, particularly for excited translational states.

We report the results of a rigorous theoretical study of the quantum translation−rotation (T−R) dynamics of one, two, and three H2 and D2 molecules confined inside the small dodecahedral (H2O)20 cage of the sII clathrate hydrate. For a single D2 molecule, o- and p-D2, in the small cage, accurate quantum five-dimensional (5D) calculations of the T−R energy levels and wave functions are performed by diagonalizing the 5D Hamiltonian which includes explicitly, as fully coupled, all three translational and the two rotational degrees of freedom of D2, while the cage is taken to be rigid. These calculations provide a quantitative description of the quantum dynamics of D2 inside the small cage and enable comparison with our quantum 5D results for the encapsulated H2, p- and o-H2, published very recently. The ground-state properties of one, two, and three p-H2 and o-D2 molecules in the small cage are calculated rigorously using the diffusion Monte Carlo method, with the emphasis on the quantum dynamics of two confined hydrogen molecules. The guest molecules are found to be effectively excluded from the sizable central region of the cage; they reside within a shell less than 2 bohrs wide and are additionally localized by the corrugation of the H2−cage interaction potential. The two H2 molecules are compressed, their mean distance inside the cage being much smaller than in the free H2 dimer.