Christian F. Schwenk

Characterization of dynamics and reactivities of solvated ions by ab initio simulations.

Based on a systematic investigation of trajectories of ab initio quantum mechanical/molecular mechanical simulations of numerous cations in water a standardized procedure for the evaluation of mean ligand residence times is proposed. For the characterization of reactivity and structure‐breaking/structure‐forming properties of the ions a measure is derived from the mean residence times calculated with different time limits. It is shown that ab initio simulations can provide much insight into ultrafast dynamics that are presently not easily accessible by experiment.

Molecular dynamics simulations of Ca2+ in water: Comparison of a classical simulation including three-body corrections and Born–Oppenheimer ab initio and density functional theory quantum mechanical/molecular mechanics simulations

A classical molecular dynamics simulation including three-body corrections was compared with combined ab initio quantum mechanics/molecular mechanics molecular dynamics simulations (QM/MM–MD), which were carried out at Hartree–Fock (HF) and density functional theory (DFT) level for Ca2+ in water. In the QM approach the region of primary interest—the first hydration sphere of the calcium ion—was treated by Born–Oppenheimer quantum mechanics, while the rest of the system was described by classical pair potentials. Coordination numbers of 7.1, 7.6, and 8.1 were found in the classical, the HF, and the DFT simulation, respectively, using the same double-ζ basis set in both QM methods. The CPU time for one DFT step was about 50% above the time for a HF step, but due to a smaller number of steps needed for equilibration in the DFT case, there was no significant difference in the overall simulation time.

Extended ab initio quantum mechanical/molecular mechanical molecular dynamics simulations of hydrated Cu2+

Copper(II) was used as a model system to investigate the relevance of including the full second hydration shell in ab initio treatment while describing hydrated ions in hybrid quantum mechanical/molecular mechanical molecular dynamics (QM/MM MD) simulations. Three different simulation techniques were applied (Hartree–Fock, B3LYP, and resolution of the identity density functional theory) to find a good compromise between accuracy and simulation speed. To discuss details of the hydration structure radial distribution functions, coordination number distributions and various angular distributions have been used. Dynamical properties such as vibrational motions of water molecules and ion–oxygen stretching motions were investigated using approximative normal coordinate analyses. QM/MM MD simulations offer a detailed time picture of the dynamic Jahn–Teller effect of Cu2+ showing short-term as well as long-term distortions to occur within <200 fs and 2–3 ps. The results prove that for transition metal ions such a...

Dynamics of the solvation process of Ca2+ in water

Abstract Combined quantum mechanics/molecular mechanics–molecular dynamics simulations (QM/MM–MD) were carried out at Hartree–Fock (HF) and density functional theory (DFT) level for Ca 2+ in water to investigate the exchange process of water molecules between first and second hydration shell. In both cases one exchange reaction took place during a simulation time of about 10 ps. The time needed for this water exchange is 1.2 ps in the DFT/MM and 2.0 ps in the QM/MM–HF simulation. From the data obtained so far, the exchange rate can be estimated to be larger than 10 −11 s.

Ab initio QM/MM MD simulations of the hydrated Ca^2^+ ion

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 ions 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.

Influence of heteroligands on structural and dynamical properties of hydrated Cu2+: QM/MM MD simulations

Combined ab initio quantum mechanical (QM)/molecular mechanical (MM) molecular dynamics (MD) simulations have been applied to mono- and diamino Cu2+ complexes in water. The simulations provide insight into the influence of one or two heteroligands in the first hydration shell on structural as well as dynamical properties of the ion. All three simulation results (monoamine, cis- and trans-diamine) show the sixfold coordinated [5 + 1] or [4 + 2] configuration to be by no means the unique species present, in the trans-diamine complex the sevenfold coordinated [Cu(H2O)5(NH3)2]2+ is even the dominating species. The effect of one or two additional ammonia molecules on the water exchange rate and the corresponding mean ligand residence time is enormous: All systems showed first shell water exchange processes within the simulation time of 20 ps, yielding mean residence times of ∼115 ps (1 NH3), ∼75 ps (2 NH3 cis) and ∼35 ps (2 NH3trans), which is faster than in pure water by a factor of 2 (monoamine), 3 (cis-diamine) and 6.5 (trans-diamine). The results also demonstrate the importance of the relative position of two heteroligands in the first shell and thus allow conclusions on the reactivity of Cu2+ N-coordinated to biologically relevant ligands.

The influence of heteroligands on the reactivity of Ni2+ in solution

Quantum-mechanics based molecular dynamics simulations were used to investigate mono-, di-, tri- and tetraamino Ni2+ complexes in water. The simulations show an enormous influence of heteroligands on the reactivity of the first solvation shell of the Ni2+ ion. Comparing 17O-NMR measurements of identical systems with our simulation results shows a 10(4) times higher mobility of water molecules in the first solvation shell obtained from QM/MM MD simulations strongly affecting biochemically important properties of Ni2+ in the aqueous environment in living organisms.

The influence of the Jahn–Teller effect and of heteroligands on the reactivity of Cu2+

Experimentally hardly accessible Jahn-Teller inversions and the influence of heteroligands on the reactivity of Cu2+ are characterized by ab initio QM/MM MD simulations of Cu2+ ion and its amino complexes in water.

Classical Versus Quantum Mechanical Simulations: The Accuracy of Computer Experiments in Solution Chemistry

Sophisticated simulation techniques in combination with high-speed computing provide a very powerful tool for the elucidation of structural data and dynamics of solutions, which in several aspects can be superior to any experimental technique. A careful analysis and comparison of simulation results achieved at different levels of accuracy shows that classical simulations, even including 3-body corrections, do not supply sufficiently precise data for all structural details and dynamical processes. As simulation techniques based on small clusters and simple density functionals also fail in the prediction of ion solvate structures, mixed quantum mechanical/molecular mechanical (QM/MM) simulations at Hartree-Fock level with medium-sized basis sets appear as the only viable method within today’s computational affordability to achieve the necessary accuracy for a theoretical approach to the details of microspecies structures and their dynamics in electrolyte solutions. Results of QM/MM-MD simulations for numerous main group and transition metal cations presented here exemplify the capability of this method and clearly show the limits not only of classical simulation techniques, but also of the models being used for the interpretation of experimental measurements.