Bernhard R. Randolf

Structure and ultrafast dynamics of liquid water: A quantum mechanics/ molecular mechanics molecular dynamics simulations study

A quantum mechanics/molecular mechanics molecular dynamics simulation was performed for liquid water to investigate structural and dynamical properties of this peculiar liquid. The most important region containing a central reference molecule and all nearest surrounding molecules (first coordination shell) was treated by Hartree-Fock (HF), post-Hartree-Fock [second-order Moller-Plesset perturbation theory (MP2)], and hybrid density functional B3LYP [Beckes three parameter functional (B3) with the correlation functional of Lee, Yang, and Parr (LYP)] methods. In addition, another HF-level simulation (2HF) included the full second coordination shell. Site to site interactions between oxygen-oxygen, oxygen-hydrogen, and hydrogen-hydrogen atoms of all ab initio methods were compared to experimental data. The absence of a second peak and the appearance of a shoulder instead in the gO-O graph obtained from the 2HF simulation is notable, as this feature has been observed so far only for pressurized or heated water. Dynamical data show that the 2HF procedure compensates some of the deficiency of the HF one-shell simulation, reducing the difference between correlated (MP2) and HF results. B3LYP apparently leads to too rigid structures and thus to an artificial slow down of the dynamics.

Structure and dynamics of phosphate ion in aqueous solution: an ab initio QMCF MD study.

A simulation of phosphate in aqueous solution was carried out employing the new QMCF MD approach which offers the possibility to investigate composite systems with the accuracy of a QMMM method but without the time consuming creation of solute–solvent potential functions. The data of the simulations give a clear picture of the hydration shells of the phosphate anion. The first shell consists of 13 water molecules and each oxygen of the phosphate forms in average three hydrogens bonds to different solvent molecules. Several structural parameters such as radial distribution functions and coordination number distributions allow to fully characterize the embedding of the highly charged phosphate ion in the solvent water. The dynamics of the hydration structure of phosphate are described by mean residence times of the solvent molecules in the first hydration shell and the water exchange rate.

An extended ab initio QM/MM MD approach to structure and dynamics of Zn"II… in aqueous solution

Structural and dynamical properties of Zn(II) in aqueous solution were investigated, based on an ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulation at double-zeta Hartree-Fock quantum mechanical level including the first and second hydration shells into the QM region. The inclusion of the second shell in the QM region resulted in significant changes in the properties of the hydrate. The first shell coordination number was found to be 6, the second shell consists of approximately 14 water molecules. The structural properties were determined in terms of RDF, ADF, tilt and theta angle distributions, while dynamics were characterized by mean ligand residence times, ion-ligand stretching frequencies and the vibrational and librational motions of water ligands.

Simulations of Liquids and Solutions Based on Quantum Mechanical Forces

Abstract Statistical mechanics simulations have become an increasingly important tool of investigation for condensed systems, in particular of aqueous and nonaqueous solutions. Especially molecular dynamics (MD) are used to obtain detailed data for structure and dynamics of solutes and thus for species formation in solution. The incorporation of quantum mechanics in MD — only possible after massive improvement of computational capacity and performance — has strongly improved the accuracy of simulations, recognizable from the excellent reproduction of experimental results. Due to the enormous computational effort associated with quantum mechanical (QM) force calculations in every step of the simulations, compromises in their technical conditions are still unavoidable. A new simulation methodology, the ab initio quantum mechanical charge field molecular dynamics (QMCF MD) procedure, has achieved significant progress in universality of application and accuracy of both structural and dynamical data. This simulation protocol is outlined in our contribution and a number of examples, ranging from simple one-atomic ions to composite ions and neutral molecules in aqueous solution, are presented to illustrate the capabilities of the method. It is shown that highly accurate simulations can in some aspects reveal details of the chemistry in solution even beyond present experimental possibilities, in particular when ultrafast dynamical processes and reactions occur. The large QM subregion of the simulation box in the QMCF MD method also allows to study reactions including formation and cleavage of bonds, illustrated here by the simulation of hydrolysis processes.

Structure and Dynamics of the U4+ Ion in Aqueous Solution: An ab Initio Quantum Mechanical Charge Field Molecular Dynamics Study

The structure and dynamics of the stable four-times positively charged uranium(IV) cation in aqueous solution have been investigated by ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) simulation at the Hartree-Fock double-zeta quantum mechanical level. The QMCF-MD approach enables investigations with the accuracy of a quantum mechanics/molecular mechanics approach without the need for the construction of solute-solvent potentials. Angular distribution functions; radial distribution functions; coordination numbers of the first, second, and third shell (9, 19, and 44, respectively); coordination number distribution functions; tilt- and Theta-angle distribution functions; as well as local density corrected triangle distribution functions have been employed for the evaluation of the hydrated ions structure. Special attention was paid to the determination of the geometry of the first hydration layer, and the results were compared to experimental large-angle X-ray scattering and extended X-ray absorption fine structure data. The solvent dynamics around the ion were also investigated using mean ligand residence times and related data and, resulting from the unavailability of any experimental data, were compared to ions with similar properties.

A Quantum Mechanical Charge Field Molecular Dynamics Study of Fe2+ and Fe3+ Ions in Aqueous Solutions

Ab initio quantum mechanical charge field molecular dynamics (QMCF-MD) simulations have been performed for aqueous solutions of Fe(2+) and Fe(3+) ions at the Hartree-Fock level of theory to describe and compare their structural and dynamical behavior. The structural features of both hydrated ions are characterized by radial distribution functions that give the maximum probability of the ion-O distance for Fe(2+) and Fe(3+) ions at 2.15 and 2.03 A, respectively. The angular distribution functions of both ions prove the octahedral arrangement of six water ligands, whereas the second shells of these ions differ. Both ions show influence on the water molecules beyond the second shells. The structure-forming abilities of both ions are visible from the ligand mean residence times and ion-O stretching frequencies evaluated for both ions. The substantially improved data obtained from these QMCF-MD simulations show better correlation with available experimental results than the conventional quantum mechanics/molecular mechanics molecular dynamics (QM/MM MD) approaches with one hydration shell treated by quantum mechanics.

Al(III) hydration revisited. An ab initio quantum mechanical charge field molecular dynamics study.

To assess the novel quantum mechanical charge field (QMCF) molecular dynamics (MD) approach, two simulations of hydrated Al(III) have been carried out, as this system proved to be a well-suited test case for hybrid ab initio/molecular mechanics simulations. Two different population analysis schemes according to Mulliken and Lowdin have been applied to evaluate the atomic charges in the QM region. It is shown that the QMCF MD approach yields a substantially improved description of the system and that, due to the fact that solute-solvent potentials can be renounced, the QMCF MD framework is a more convenient approach to investigate solvated systems compared to conventional ab initio QM/MM MD approaches.

Influence of polarization and many body quantum effects on the solvation shell of Al(III) in dilute aqueous solution--extended ab initio QM/MM MD simulations.

Structural properties of the hydrated Al(III) ion have been investigated by ab initio quantum mechanical/ molecular mechanical (QM/MM) molecular dynamics (MD) simulations at double zeta HF quantum mechanical level including only the first and first plus second hydration shell into the QM region. The coordination number in the first shell was found to be 6.0 in both cases, but the inclusion of the second shell into the QM region causes significant changes in the properties of the hydrate. Several structural parameters such as angular distribution functions, radial distribution functions and tilt- and theta-angle distributions were used to fully characterise the hydration structure of Al(III).

Structure-breaking effects of solvated Rb(I) in dilute aqueous solution—An ab initio QM/MM MD approach

Structural properties of the hydrated Rb(I) ion have been investigated by ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) simulations at the double‐zeta HF quantum mechanical level. The first shell coordination number was found to be 7.1, and several other structural parameters such as angular distribution functions, radial distribution functions and tilt‐ and θ‐angle distributions allowed the full characterization of the hydration structure of the Rb(I) ion in dilute aqueous solution. Velocity autocorrelation functions were used to calculate librational and vibrational motions, ion–ligand motions, as well as reorientation times. Different dynamical parameters such as water reorientation, mean ligand residence time, the number of ligand exchange processes, and rate constants were also analyzed. The mean ligand residence time for the first shell was determined as τ = 2.0 ps.

Structure and Dynamics of the UO22+ Ion in Aqueous Solution: An Ab Initio QMCF MD Study

A comprehensive theoretical investigation on the structure and dynamics of the UO(2)(2+) ion in aqueous solution using double-zeta HF level quantum mechanical charge field molecular dynamics is presented. The quantum mechanical region includes two full layers of hydration and is embedded in a large box of explicitly treated water to achieve a realistic environment. A number of different functions, including segmential, radial, and angular distribution functions, are employed together with tilt- and Theta-angle distribution functions to describe the complex structural properties of this ion. These data were compared to recent experimental data obtained from LAXS and EXAFS and results of various theoretical calculations. Some properties were explained with the aid of charge distribution plots for the solute. The solvent dynamics around the ion were investigated using distance plots and mean ligand residence times and the results compared to experimental and theoretical data of related ions.