Round Robin Study: Molecular Simulation of Thermodynamic Properties from Models with Internal Degrees of Freedom.

Thermodynamic properties are often modeled by classical force fields which describe the interactions on the atomistic scale. Molecular simulations are used for retrieving thermodynamic data from such models, and many simulation techniques and computer codes are available for that purpose. In the present round robin study, the following fundamental question is addressed: Will different user groups working with different simulation codes obtain coinciding results within the statistical uncertainty of their data? A set of 24 simple simulation tasks is defined and solved by five user groups working with eight molecular simulation codes: DL_POLY, GROMACS, IMC, LAMMPS, ms2, NAMD, Tinker, and TOWHEE. Each task consists of the definition of (1) a pure fluid that is described by a force field and (2) the conditions under which that property is to be determined. The fluids are four simple alkanes: ethane, propane, n-butane, and iso-butane. All force fields consider internal degrees of freedom: OPLS, TraPPE, and a modified OPLS version with bond stretching vibrations. Density and potential energy are determined as a function of temperature and pressure on a grid which is specified such that all states are liquid. The user groups worked independently and reported their results to a central instance. The full set of results was disclosed to all user groups only at the end of the study. During the study, the central instance gave only qualitative feedback. The results reveal the challenges of carrying out molecular simulations. Several iterations were needed to eliminate gross errors. For most simulation tasks, the remaining deviations between the results of the different groups are acceptable from a practical standpoint, but they are often outside of the statistical errors of the individual simulation data. However, there are also cases where the deviations are unacceptable. This study highlights similarities between computer experiments and laboratory experiments, which are both subject not only to statistical error but also to systematic error.

Comparative study of the Grüneisen parameter for 28 pure fluids.

The Grüneisen parameter γG is widely used for studying thermal properties of solids at high pressure and also has received increasing interest in different applications of non-ideal fluid dynamics. Because there is a lack of systematic studies of the Grüneisen parameter in the entire fluid region, this study aims to fill this gap. Grüneisen parameter data from molecular modelling and simulation are reported for 28 pure fluids and are compared with results calculated from fundamental equations of state that are based on extensive experimental data sets. We show that the Grüneisen parameter follows a general density-temperature trend and characterize the fluid systems by specifying a span of minimum and maximum values of γG. Exceptions to this trend can be found for water.

Assessing the accuracy of improved force-matched water models derived from Ab initio molecular dynamics simulations.

The accuracy of water models derived from ab initio molecular dynamics simulations by means on an improved force-matching scheme is assessed for various thermodynamic, transport, and structural properties. It is found that although the resulting force-matched water models are typically less accurate than fully empirical force fields in predicting thermodynamic properties, they are nevertheless much more accurate than generally appreciated in reproducing the structure of liquid water and in fact superseding most of the commonly used empirical water models. This development demonstrates the feasibility to routinely parametrize computationally efficient yet predictive potential energy functions based on accurate ab initio molecular dynamics simulations for a large variety of different systems. © 2016 Wiley Periodicals, Inc.

Communication: Molecular simulation study of kaolinite intercalation with realistic layer size.

Intercalation phenomena of kaolinite in aqueous potassium acetate and in hexyl-amine solutions are studied by large scale molecular dynamics simulations. The simulated kaolinite particle is constructed from ~6.5 × 10(6) atoms, producing a particle size of ~100 nm × 100 nm × 10 nm. The simulation with potassium acetate results in a stable kaolinite-potassium acetate complex, with a basal spacing that is in close agreement with experimental data. The simulation with hexyl-amine shows signs of the experimentally observed delamination of kaolinite (the initial phase of the formation of nanoscrolls from the external layers).

Simulation-assisted evidence for the existence of two stable kaolinite/potassium acetate intercalate complexes.

Recent molecular simulation findings with several kaolinite intercalate complexes raised the question of the existence of more than one stable state, which has not been confirmed by experimental observations yet. Kaolinite/potassium acetate intercalate complexes were synthesized and examined by X-ray diffraction, and a molecular simulation study was performed for the system. Consistent with the suggestion from the simulations, an additional stable basal spacing was found experimentally at d(001)=1.168nm besides the well-known one at d(001)=1.403nm.

Dynamic Monte Carlo simulation in mixtures.

The dynamic Monte Carlo technique is a widely used simulation tool but the parameters of the calculation have to be tuned to reflect the same dynamics as the corresponding molecular dynamics simulation. As the direct calibration of the dynamic Monte Carlo with molecular dynamics is a laborious task, we propose a new method that allows the standard dynamic Monte Carlo to realize the correct time proportionality in many-component systems without the need of corresponding molecular dynamics calculation. The method has been tested in various systems and the dynamic Monte Carlo results obtained by the proposed method were found to be in good agreement with the results of the control molecular dynamics simulations.

Simulation and experimental study of intercalation of urea in kaolinite.

Experimental measurements and molecular simulations were used to describe the characteristics of the kaolinite/urea intercalation compound. The intercalation compound was synthesized by a mechanochemical method and examined by X-ray diffraction and thermogravimetry. Additionally, a series of NpT (constant particle number-pressure-temperature) simulations was performed to identify thermodynamically stable basal spacings. From the simulations the most probable molecular orientations were determined for single and double layered arrangements of urea molecules that develop between the layers of kaolinite.