S. Vandenbrande

Knowledge of the thermodynamic potential in terms of the independent variables allows to characterize the macroscopic state of the system. However, in practice, it is difficult to access this potential experimentally due to irreversible transitions that occur between equilibrium states. A showcase example of sudden transitions between (meta)stable equilibrium states is observed for soft porous crystals possessing a network with long-range structural order, which can transform between various states upon external stimuli such as pressure, temperature and guest adsorption. Such phase transformations are typically characterized by large volume changes and may be followed experimentally by monitoring the volume change in terms of certain external triggers. Herein, we present a generalized thermodynamic approach to construct the underlying Helmholtz free energy as a function of the state variables that governs the observed behaviour based on microscopic simulations. This concept allows a unique identification of the conditions under which a material becomes flexible.

The search for nanoporous materials that are highly performing for gas storage and separation is one of the contemporary challenges in material design. The computational tools to aid these experimental efforts are widely available and adsorption isotherms are routinely computed for huge sets of (hypothetical) frameworks. Clearly the computational results depend on the interactions between the adsorbed species and the adsorbent, which are commonly described using force fields. In this paper, an extensive comparison and in-depth investigation of several force fields from literature is reported for the case of methane adsorption in the Zr-based Metal-Organic Frameworks UiO-66, UiO-67, DUT-52, NU-1000 and MOF-808. Significant quantitative differences in the computed uptake are observed when comparing different force fields, but most qualitative features are common which suggests some predictive power of the simulations when it comes to these properties. More insight into to the host-guest interactions is obtained by benchmarking the force fields with an extensive number of ab initio computed single molecule interaction energies. This analysis at the molecular level reveals that especially ab initio derived force fields perform well in reproducing the ab initio interaction energies. Finally, the high sensitivity of uptake predictions on the underlying potential energy surface is explored.

We propose a methodology to derive pairwise-additive noncovalent force fields from monomer electron densities without any empirical input. Energy expressions are based on the symmetry-adapted perturbation theory (SAPT) decomposition of interaction energies. This ensures a physically motivated force field featuring an electrostatic, exchange repulsion, dispersion, and induction contribution, which contains two types of parameters. First, each contribution depends on several fixed atomic parameters, resulting from a partitioning of the monomer electron density. Second, each of the last three contributions (exchange-repulsion, dispersion, and induction) contains exactly one linear fitting parameter. These three so-called interaction parameters in the model are initially estimated separately using SAPT reference calculations for the S66x8 database of noncovalent dimers. In a second step, the three interaction parameters are further refined simultaneously to reproduce CCSD(T)/CBS interaction energies for the same database. The limited number of parameters that are fitted to dimer interaction energies (only three) avoids ill-conditioned fits that plague conventional parameter optimizations. For the exchange repulsion and dispersion component, good results are obtained for all dimers in the S66x8 database using one single value for the associated interaction parameters. The values of those parameters can be considered universal and can also be used for dimers not present in the original database used for fitting. For the induction component such an approach is only viable for the dispersion dominated dimers in the S66x8 database. For other dimers (such as hydrogen-bonded complexes), we show that our methodology remains applicable. However, the interaction parameter needs to be determined on a case-specific basis. As an external validation:, the force field predicts interaction energies in good agreement with CCSD(T)/CBS values for dispersion dominated dimers extracted from an HIV-II protease crystal structure with a bound ligand (indinavir). Furthermore, experimental second virial coefficients of small alkanes and alkenes are well reproduced.