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.

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A systematic study of the effect of coordinatively unsaturated sites (cus) in the separation of CO2/CH4 and CO/CO2/CH4 mixtures on CPO-27-M (M = Ni, Co, and Zn) and STA-12-Ni metal–organic frameworks was carried out using gravimetric and breakthrough experiments. The separation selectivity and the working capacity of these structures were evaluated as important performance indicators for CO2 separations by PSA. The results demonstrate a remarkable influence of coordinatively unsaturated sites on the selectivity and the working capacity. Particularly, the high affinity of CPO-27-Ni and CPO-27-Co for CO2 leads to a low working capacity for CO2 (because regeneration of the adsorbents is difficult) but a high CO2/CH4 selectivity. With a ternary CO/CO2/CH4 feed mixture, CPO-27-Ni and -Co prefer the adsorption of CO over CO2 due to the strong specific interaction of CO with cus. Surprisingly, STA-12-Ni does not exhibit the same behavior: it is selective for the adsorption of CO2 in a ternary mixture, just like CPO-27-Zn. Among the four MOFs tested in this study, CPO-27-Zn presents the best compromise between the working capacity and the CO2/CH4 and CO2/CO selectivities. The results are discussed in terms of the coordination chemistry of the coordinatively unsaturated metal sites, their acid–base properties, and their accessibility.

The choice of an appropriate adsorbent for CO 2 separation by pressure-swing adsorption remains a field of intense research. In this work, several FAU and LTA zeolites with different Na contents (Si/Al ratios) are studied for the separation of CO 2 from mixtures of CO 2 , CO, and CH 4 by means of breakthrough experiments. The breakthrough experiments were carried out between 1 and 5 bar at 303 K using two feed mixtures: 50/50 (v/v) CO 2 /CH 4 and 75/15/15 (v/v/v) CO 2 / CH 4 /CO. The most polar zeolites, i.e., those with high Na content, exhibit the highest adsorption capacity and selectivity for CO 2 , but their regeneration is difficult; hence, their working capacity is low. The opposite is true for the least polar zeolites, i.e., those with low Na content. In order to quantify the trade-off between the selectivity and working capacity, the Ruthven statistical model (RSM) was used. It satisfactorily reproduced the experimental trends. We, therefore, used the RSM to identify the properties of the adsorbent that lead to an optimal compromise between the working capacity and separation factor. The critical parameter is the concentration of extraframework cations, which, in turn, depends on the framework charge of the zeolites FAU and LTA. The optimal trade-off zone is defined in terms of the Henry constant of CO 2 (K CO 2 ). It is found that this zone is placed between K CO 2 = 5 × 10 −3 and 50 × 10 −3 molecules•bar −1 •Å −3 . This interval corresponds to a heat of adsorption of CO 2 at zero coverage between 27 and 32 kJ•mol −1 . In our study, this optimal range of Henry constants was achieved for the zeolites Na-USY, SAPO-37, LTA (Si/Al = 5), and EMC-1.

We propose a method for analytically predicting single-component adsorption isotherms from molecular, microscopic and structural descriptors of the adsorbate-adsorbent system and concepts of statistical thermodynamics. Expressions for Henry's constant and the heat of adsorption at zero coverage are derived. These functions depend on the pore size, pore shape, chemical composition, and density of the adsorbent material. They quantify the strength of the solid-fluid interaction, which governs the low-pressure part of the adsorption isotherm. For intermediate and high pressures, the fluid-fluid interactions must also be taken into account. Both solid-fluid and fluid-fluid interactions are combined within the framework of the Ruthven statistical model (RSM). The RSM thus constructs theoretical adsorption isotherms that are entirely based on microscopic molecular and structural descriptors. The theoretical results that we obtained are compared with experimental data for the adsorption of pure CO2 and CH4 on all-silica zeolites. The developed methodology allows for the estimation of the optimum properties of a nonpolar adsorbent for the adsorption of CO2 in cyclic adsorption processes.

It is well known that studying equilibria of polymers in solution by atomistic simulations is computationally demanding as a large phase space has to be adequately sampled. Nevertheless, direct molecular dynamics (MD) simulations are often used for this purpose in the literature. To assess whether such approach is adequate, we have conducted a case study for a polymer + solvent system that has been commonly studied with direct MD simulations by many authors: poly(nisopropylacrylamide) (PNIPAM) in water. The total simulation time of the present study is much longer than that typically used in MD simulations of that system. A NIPAM chain of 30 monomers was studied in explicit water at 295 K. Three initial configurations were used. For each configuration, five replicas were run for 1000 ns. The statistical analysis of our data shows that the equilibration time is of the order of 600 -700 ns and that the remaining time for the production run is not sufficient to sample the equilibrium state adequately. These results underpin the well-known difficulty of sampling equilibrium states of polymers in solution with direct MD simulations and the need for a careful interpretation of results of such studies. The problem with the unsatisfactory sampling persists despite the increasing available computing power. Therefore, enhanced sampling techniques and workarounds, such as simplified scenarios or coarse-graining, remain important.

Studying equilibrium properties of polymers in solution by atomistic simulations is a challenging task as the available computation time is often not sufficient to ensure representative sampling of the phase space. One approach to tackle this problem is to create a simulation scenario which is simple enough to enable adequate sampling of equilibrium states while it retains the essential parts of the physics of the polymer in solution. In this work, we present and test such a scenario, which is designed for studying whether a given

The confinement effect plays a key role in physisorption in microporous materials and many other systems. Confinement is related to the relationship between the pore geometry (pore size and topology) and the geometry of the adsorbed molecule. Geometric properties of the porous solid can be described using the concepts of Gaussian and mean curvatures. In this work we show that the Gaussian and mean curvatures are suited descriptors for mathematically quantifying the confinement of small molecules in porous solids. A method to determine these geometric parameters on microporous materials is presented. The new methodology is based on the reconstruction of the solid's accessible surface. Then, a numerical calculation of the Gaussian and mean curvatures is carried out over the reconstructed mesh. On the one hand, we show that the local curvature can be used to identify the most favourable adsorption sites. On the other hand, the global mean curvature of the solid is correlated to the heat of adsorption of CO2 and CH4 on several zeolites and MOFs. A theoretical justification for this empirical correlation is provided. In conclusion, our methodology allows for a semi-quantitative estimation of confinement, applicable to any pore geometry, independent of the chemical composition, and without the need for applying a force field.