Probing the range of applicability of structure- and energy-adjusted QM/MM link bonds III: QM/MM MD simulations of solid-state systems at the example of layered carbon structures

The previously introduced workflow to achieve an energetically and structurally optimized description of frontier bonds in quantum mechanical/molecular mechanics (QM/MM)-type applications was extended into the regime of computational material sciences at the example of a layered carbon model systems. Optimized QM/MM link bond parameters at HSEsol/6-311G(d,p) and self-consistent density functional tight binding (SCC-DFTB) were derived for graphitic systems, enabling detailed investigation of specific structure motifs occurring in graphene-derived structures v i a quantum-chemical calculations. Exemplary molecular dynamics (MD) simulations in the isochoric-isothermic (NVT) ensemble were carried out to study the intercalation of lithium and the properties of the Stone-Thrower-Wales defect. The diffusivity of lithium as well as hydrogen and proton adsorption on a defective graphene surface served as additional example. The results of the QM/MM MD simulations provide detailed insight into the applicability of the employed link-bond strategy when studying intercalation and adsorption properties of graphitic materials.

Towards hybrid quantum mechanical/molecular mechanical simulations of Li and Na intercalation in graphite - force field development and DFTB parametrisation

In this work a previously established QM/MM simulation protocol for the treatment of solid-state interfaces was extended towards the treatment of layered bulk materials enabling for instance investigation of metal intercalation in graphitic carbon materials. In order to study the intercalation of Li in graphite, new density functional tight binding (DFTB) parameters for Li have been created. Molecular dynamics (MD) simulations at constant temperatures (273.15, 298.15 and 323.15 K) have been carried out to assess the performance of the presented DFTB MD simulation approach. The intercalation of variable lithium and sodium content was investigated via z-distribution functions and analysis of the diffusivity in the direction parallel to the graphene plane. Both the calculated diffusion coefficients and the activation energy in case of lithium are in good agreement with experimental data. The comparison of the QM/MM MD simulation results provide detailed insights into the structural and dynamical properties of intercalated metal ions.

Why halides enhance heterogeneous metal ion charge transfer reactions

The reaction kinetics of many metal redox couples on electrode surfaces are enhanced in the presence of halides (i.e., Cl-, Br-, I-). Using first-principles metadynamics simulations, we show a correlation between calculated desorption barriers of V3+-anion complexes bound to graphite via an inner-sphere anion bridge and experimental V2+/V3+ kinetic measurements on edge plane pyrolytic graphite in H2SO4, HCl, and HI. We extend this analysis to V2+/V3+, Cr2+/Cr3+, and Cd0/Cd2+ reactions on a mercury electrode and demonstrate that reported kinetics in acidic electrolytes for these redox couples also correlate with the predicted desorption barriers of metal-anion complexes. Therefore, the desorption barrier of the metal-anion surface intermediate is a descriptor of kinetics for many metal redox couple/electrode combinations in the presence of halides. Knowledge of the metal-anion surface intermediates can guide the design of electrolytes and electrocatalysts with faster kinetics for redox reactions of relevance to energy and environmental applications.

Molecular Dynamics Simulation Studies on Structure, Dynamics, and Thermodynamics of Uranyl Nitrate Solution at Various Acid Concentrations

The structural and dynamical characteristics of uranyl ions in an aqueous acidic environment are of immense importance in the field of nuclear fuel reprocessing. In view of that, the structural and dynamical behavior of the uranyl ion in water has been investigated by performing molecular dynamics (MD) simulations using different force fields. All the force fields have depicted similar structural and dynamical properties except the free energy of hydration where the Guilbaud-Wipff (GW) model performs well over the others. The calculated density using MD simulations is found to be in excellent agreement with the measured experimental density, which ensures the accuracy of the adopted GW force field. The calculated surface tension and shear viscosity are seen to be increased with uranyl nitrate concentrations. At a higher concentration of about 4.0 mol/L, the supersaturation effect has been captured by an inflection in the plot of surface tension and shear viscosity against concentration because of the solution heterogeneity, which was correlated by an inflection in the scattering intensity observed by performing the dynamic light scattering experiment. The binding mode of nitrate ions with the uranyl ion is found to be concentration-dependent, and at higher concentration, it is predominantly monodentate.

Density functional theory (DFT) calculations of VI/V reduction potentials of uranyl coordination complexes in non-aqueous solutions

Of particular interest within the +6 uranium complexes is the linear uranyl(vi) cation and it forms numerous coordination complexes in solution and exhibits incongruent redox behavior depending on coordinating ligands. In this study, to determine the reduction potentials of uranyl complexes in non-aqueous solutions, a hybrid density functional theory (DFT) approach was used in which two different DFT functionals, B3LYP and M06, were applied. Bulk solvent effects were invoked through the conductor-like polarizable continuum model. The solute cavities were described with the united-atom Kohn-Sham (UAKS) cavity definition. Inside the cavity the dielectric constant matches the value of a vacuum and outside the cavity the dielectric constant value is the same as that of the solvent of interest, for example, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dichloromethane (DCM), acetonitrile and pyridine. With the help of the Nernst equation the calculated reduction potentials with respect to the ferrocene (Fc) reference electrode are converted into reduction free energies (RFEs). Uranyl complexes of organic ligands which range from mono- to hexa-dentate coordination modes were investigated in non-aqueous solutions of DMSO, DMF, DCM, acetonitrile and pyridine solutions. The effect of the spin-orbit correction and the reference electrode correction on the RFEs and various methods such as the direct method and the isodesmic reaction model were explored. Overall, our computational determination of RFEs of uranyl complexes in various non-aqueous solutions demonstrates that the RFEs can be obtained within ∼0.2 eV of experimental values.

Acidity constants and redox potentials of uranyl ions in hydrothermal solutions

We report a first principles molecular dynamics (FPMD) study of the structures, acidity constants (pK_a) and redox potentials (E⁰) of uranyl (UO₂²⁺) from ambient conditions to 573 K.

Molecular dynamics simulation study of distribution and dynamics of aqueous solutions of uranyl ions: the effect of varying temperature and concentration

Investigating the characteristics of actinyl ions has been of great interest due to their direct relevance in the nuclear fuel cycle. All-atom molecular dynamics simulations have been employed to study the orientational structure and dynamics of aqueous solutions of uranyl ions of various concentrations. The orientational structure of water around a uranyl ion has been thoroughly investigated by calculating different orientational probability distributions corresponding to different molecular axes of water. The orientational distribution of water molecules in the first coordination shell of a uranyl ion is found to be markedly different from that in bulk water. Analysis of counterion distribution around the uranyl ion reveals the presence of nitrate ions along with water molecules in the first solvation shell. From the comparison of the number of coordinated water and nitrate ions at various uranyl nitrate concentrations, it is evident that these two species compete for occupying the first solvation shell of the uranyl ion. Orientational dynamics of water molecules about different molecular axes of water in the vicinity of uranyl ions have also been investigated and decreasing orientational mobility of water with increasing uranyl concentration has been found. However, it is observed that the orientational dynamics remains more or less the same whether we consider all the water molecules in the aqueous solution or only the solvation shell water molecules. The effect of temperature on the translational and orientational characteristics of the aqueous uranyl solutions has also been studied in detail.

Perspectives for hybrid ab initio/molecular mechanical simulations of solutions: from complex chemistry to proton-transfer reactions and interfaces

As a consequence of the ongoing development of enhanced computational resources, theoretical chemistry has become an increasingly valuable field for the investigation of a variety of chemical systems. Simulations employing a hybrid quantum mechanical/molecular mechanical (QM/MM) molecular dynamics (MD) technique have been shown to be a particularly promising approach, whenever ultrafast (i.e., picosecond) dynamical properties are to be studied, which are in many cases difficult to access via experimental techniques. Details of the quantum mechanical charge field (QMCF) ansatz, an advanced QM/MM protocol, are discussed and simulation results for various systems ranging from simple ionic hydrates to solvated organic molecules and coordination complexes in solution are presented. A particularly challenging application is the description of proton-transfer reactions in chemical simulations, which is a prerequisite to study acidified and basic systems. The methodical requirements for a combination of the QMCF methodology with a dissociative potential model for the description of the solvent are discussed. Furthermore, the possible extension of QM/MM approaches to solid/liquid interfaces is outlined.