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In the quest towards coarse-grained potentials and new water models, we present an extension of the force matching technique to parameterize an all-atom force field for rigid water. The methodology presented here allows to improve the matching procedure by first optimizing the weighting exponents present in the objective function. A new gauge for unambiguously evaluating the quality of the fit has been introduced; it is based on the root mean square difference of the distributions of target properties between reference data and fitted potentials. Four rigid water models have been parameterized; the matching procedure has been used to assess the role of the ghost atom in TIP4P-like models and of electrostatic damping. In the former case, burying the negative charge inside the molecule allows to fit better the torques. In the latter, since short-range interactions are damped, a better fit of the forces is obtained. Overall, the best performing model is the one with a ghost atom and with electrostatic damping. The approach shown in this paper is of general validity and could be applied to any matching algorithm and to any level of coarse graining, also for non-rigid molecules. [ABSTRACT FROM AUTHOR]

The optimized geometry, energetics, and vibrational properties of Al(D2O)n3+ clusters, with n=1,2,4, and 6, have been studied using plane waves, different local basis sets, different methodologies [density-functional theory, MP2, CCSD(T)], and different functionals (BLYP, PBE). Moreover, Car-Parrinello molecular-dynamics (MD) simulations using the BLYP functional, plane waves, and the Vanderbilt ultrasoft pseudopotentials have been performed for an aqueous Al3+ solution with 1 ion and 32 D2O molecules in a periodic box at room temperature, studied for 10 ps. The cluster calculations were performed to pinpoint possible shortcomings of the electronic structure description used in the Car-Parinello MD (CPMD) simulation. For the clusters, the hydration structure and interaction energies calculated with the ‘BLYP/plane-wave’ approach agree well with high-level ab initio methods but the exchange-correlation functional introduces errors in the OD stretching frequencies (both in the absolute values and in the ion-induced shifts). For the aqueous solution, the CPMD simulation yields structural properties in good agreement with experimental data. The CPMD-simulated OD stretching vibrational band for the first-shell water molecules around Al3+ is strongly downshifted by the influence of the ion and is compared with experimental data from the literature. To make such a comparison meaningful, the influences of a number of systematic effects have been addressed, such as the exchange-correlation functional, the fictitious electron mass, anharmonicity effects, and the small box size in the simulation. Each of these factors (except the last one) is found to affect the OD frequency by 100 cm-1 or more. The final “corrected” frequencies agree with experiment within ∼30 cm-1 for bulk water but are too little downshifted for the first-shell Al3+(aq) water molecules (by ∼200 cm-1). [ABSTRACT FROM AUTHOR]

Abstract: Recently many various research groups have devoted a huge effort to develop a realistic classical force field for ions in water. The parametrization techniques used could be gathered into two classes: (i) fit of the ab initio potential energy surface for clusters at gas phase, and (ii) fit of experimental properties. For both classes of force fields, a high level of accuracy has been achieved, which has led to important improvements in the modeling of ion–water systems. In this paper a new, complementary, approach is proposed to overcome the limitations and to get a deeper insight into the atomistic description of ion–water interactions. We use the recently developed force matching method to parametrize classical halide–water force fields for three different water models. Here we discuss both methodological issues and the level of agreement between the results obtained using this method to Car–Parrinello simulation results. [Copyright &y& Elsevier]

Abstract: This work presents a ReaxFF reactive force-field for use in molecular dynamics simulations of the ZnO–water system. The force-field parameters were fitted to a data-set of energies, geometries and charges derived from quantum-mechanical B3LYP calculations. The presented ReaxFF model provides a good fit to the QM reference data for the ZnO–water system that was present in the data-set. The force-field has been used to study how water is adsorbed, molecularly or dissociatively, at monolayer coverage on flat and stepped ZnO surfaces, at three different temperatures (10 K, 300 K, and 600 K). The stepped surfaces were created by introducing steps along the (0001)-direction on the -surface. Equilibrium between molecular and dissociated water was observed on the terraces, resulting in a half dissociated, half molecular water monolayer. The equilibrium between dissociated and molecular water on the surface was found to be reached quickly (<10ps). When water molecules desorb and the coverage falls, the 1:1 water–hydroxyl ratio is maintained on terraces, while steps remain largely hydroxylated. The results show that structures that promote hydrogen bonding are favored and that the presence of steps promotes an increased level of hydroxylation in the water monolayers. [Copyright &y& Elsevier]