We have tested the ability of two new model potentials constructed using intermolecular perturbation theory methods to reproduce ab initio results at a comparable level of theory. Several configurations of water trimer, tetramer, and pentamer are studied, and in addition to the contributions to the interaction energy, the potential energy surfaces are compared by optimizing the model potential geometries to local stationary points within a rigid-body framework. In general the agreement between the two methods is good, validating the model potentials as suitable candidates for providing starting geometries for further ab initio calculations and for the simulation of larger systems.
Two new parametrizations of a recent ab initio
polarizable anisotropic site potential for water are
presented.
The new versions improve the description of the electrostatic
interactions, add an explicit charge-transfer
term, and use more accurate dispersion coefficients from the recent
literature. To assess the merits of the
new models, the potential energy surface of the dimer is analyzed and a
comparison is made with 12 other
polarizable potentials for water in the literature, most of them being
currently used in computer simulation.
The structure, energy, and harmonic intermolecular frequencies of
the stationary points have been determined
and compared with the best available ab initio calculations. The
energy barriers and pathways for hydrogen
atom interchange within the dimer are discussed. The second virial
coefficient B(T) of steam between
373
and 973 K, including first-order quantum corrections, is reported.
For all the models, the quantum corrections
are found to be significant at the lowest temperatures, amounting to
10−15% at 373 K. Roughly 90% of the
quantum corrections arise from the rotational degrees of freedom.
Among the potentials considered, only
those presented in the present work and a few others are really
successful in reproducing the experimental
results for B(T) in that temperature
range.
We construct a rigid-body (five-dimensional) potential-energy surface for the water-hydrogen complex using scaled perturbation theory (SPT). An analytic fit of this surface is obtained, and, using this, two minima are found. The global minimum has C2v symmetry, with the hydrogen molecule acting as a proton donor to the oxygen atom on water. A local minimum with Cs symmetry has the hydrogen molecule acting as a proton acceptor to one of the hydrogen atoms on water, where the OH bond and H2 are in a T-shaped configuration. The SPT global minimum is bound by 1097 microEh (Eh approximately 4.359744 x 10(-18) J). Our best estimate of the binding energy, from a complete basis set extrapolation of coupled-cluster calculations, is 1076.1 microEh. The fitted surface is used to calculate the second cross virial coefficient over a wide temperature range (100-3000 K). Three complementary methods are used to quantify quantum statistical mechanical effects that become significant at low temperatures. We compare our results with experimental data, which are available over a smaller temperature range (230-700 K). Generally good agreement is found, but the experimental data are subject to larger uncertainties.
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