OD vibrations and hydration structure in an Al3+ (aq) solution from a Car-Parrinello molecular-dynamics simulation

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 approximately 30 cm(-1) for bulk water but are too little downshifted for the first-shell Al3+ (aq) water molecules (by approximately 200 cm(-1)).