Ab initio QM/MM MD simulations of the hydrated Ca2+ ion

Abstract

The comparison of two different combined quantum mechanical (QM)/molecular mechanical (MM) simulations treating the quantum mechanical region at Hartree-Fock (HF) and B3-LYP density functional theory (DFT) level allowed us to determine structural and dynamical properties of the hydrated calcium ion. The structure is discussed in terms of radial distribution functions, coordination number distributions, and various angular distributions and the dynamical properties, as librations and vibrations, reorientational times and mean residence times were evaluated by means of velocity autocorrelation functions. The QM/MM molecular dynamics (MD) simulation results prove an eightfold-coordinated complex to be the dominant species, yielding average coordination numbers of 7.9 in the HF and 8.0 in the DFT case. Structural and dynamical results show higher rigidity of the hydrate complex using DFT. The high instability of calcium ion's hydration shell allows the observation of water-exchange processes between first and second hydration shell and shows that the mean lifetimes of water molecules in this first shell (<100 ps) have been strongly overestimated by conclusions from experimental data.

Temperature dependence of structure and dynamics of the hydrated Ca2+ ion according to ab initio quantum mechanical charge field and classical molecular dynamics

“…This is contrary to the experimental results of Lindgren et al, using neutron diffraction and theoretical calculations,27 where Ca 2+ was suggested to belong to the family of divalent cationic species capable of inducing tetrahedral coordination configuration of water molecules (θ ≈ 55°) within its primary hydration sphere. However, the present study shows a more relaxed dipole alignment behaviour of first shell water molecules than what was reported by Schwenk and co‐workers in a study using QM/MM MD28 (a θ value of ∼16°). To check whether the observed slightly large first hydration shell was due to the inadequacy of the level of theory used to describe the interactions in the QM region, bond energies, and bond distances from cluster calculations were inspected to see if a higher level of theory, MP2, would produce values that are different from HF.…”

A Quantum Chemical Study on Hydration of Ra (II): Comparison with the Other Hydrated Divalent Alkaline Earth Metal Ions

“…The hydration numbers of the divalent alkaline earth metal ions have been experimentally reported to be 6.0 [4][5][6], 6.0-8.0 [7][8][9], 7.3-15.0 [10][11][12][13], and 7.8-9.5 [14,15] for Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ , respectively. These results have also been well reproduced by theoretical methods [12,14,[16][17][18][19][20][21][22][23][24]. Taking into account the hydration numbers for the other alkaline earth metal ions and the atomic radii of Ra, we could expect the hydration number of Ra 2+ to be similar to that of Ba 2+ .…”

Modelling the effects of salt solutions on the hydration of calcium ions

“…For example, the formation of calcite (CaCO 3 ) directly from natural solution (ion-rich solutions such as sea water and pore water) occurs during the process of biomineralization, 3 but is also one of the most viable routes for carbon sequestration. [7][8][9][10][11][12][13][14][15] However, in natural and industrial aqueous environments the crystallization of inorganic materials occurs in the presence of non-lattice ions, such as Na + and Cl À , and consequently the solutions from which calcium-bearing crystals usually nucleate and then grow are far from pure water. 6 The properties of the hydrated calcium ion have been investigated previously using both experimental and computational approaches.…”