Ab initio QM/MM MD simulations of the hydrated Ca2+ ion
C. F. Schwenk and B. M. Rode
Department of Theoretical Chemistry, Institute of
General,Inorganic and
Theoretical Chemistry, University of Innsbruck, Innrain 52a, A-6020
Innsbruck, Austria
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.
*Lecture presented at the European Molecular Liquids
Group (EMLG) Annual Meeting on the Physical Chemistry of Liquids: Novel
Approaches to the Structure, Dynamics of Liquids: Experiments, Theories,
and Simulation,Rhodes, Greece, 7-15 September 2002. Other presentations
are published in this issue, pp. 1-261.
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