We present a general methodology for generating accurate and transferable ab initio force fields, employing the framework of symmetry adapted perturbation theory (SAPT). The resulting force fields are "physically-motivated" in that they contain separate, explicit terms to account for the various fundamental intermolecular interactions, such as exchange, electrostatics, induction, and dispersion, with each term parametrized to a corresponding term in the SAPT energy decomposition. Crucially, the resulting force fields are largely compatible with existing, standard simulation packages, requiring only minimal modifications. We present several novel parametrization techniques that yield robust, physically meaningful atomic parameters that are transferable between molecular environments. We demonstrate the accuracy and generality of our method by validating against experimental second virial coefficients for a variety of small molecules. We then show that the resulting atomic parameters can be combined using physically motivated ansatzes to accurately predict arbitrary heteromolecular interaction energies, with example applications including prediction of gas adsorption in functionalized metal-organic framework materials.

We present an entirely ab initio methodology, based on symmetry adapted perturbation theory (SAPT), for constructing force-fields to study CO 2 adsorption in nanoporous zeolitic imidazolate frameworks (ZIFs). Our approach utilizes the SAPT energy decomposition to generate physically motivated force fields for the CO 2 -ZIF interaction, with explicit terms representing exchange, electrostatic, induction and dispersion interactions. Each of these terms is fit to the corresponding term in the SAPT energy decomposition, yielding a force field entirely free of empirical parameters. This approach was utilized to construct force fields describing the CO 2 interaction with both ZIF-8 and ZIF-71. In conjunction with our existing CO 2 ÀCO 2 force field, parametrized in a consistent manner, we validate our force fields using grand canonical Monte Carlo simulations and obtain good agreement with the corresponding experimental CO 2 adsorption isotherms. Furthermore, the explicit correspondence between force field terms and fundamental interaction types (dispersion, electrostatics, and induction) allows for an analysis of the underlying physics controlling ZIF gas adsorption that is far more direct and well-defined than with the generic force fields that had been previously utilized to study these systems. As our force fields are free from empirical parameters, these results demonstrate the potential for computationally screening novel ZIFs for flue gas separation applications with near quantitative accuracy.

We extend our existing methodology for generating physically motivated, tailored ab initio force fields via symmetry-adapted perturbation theory (SAPT). The revised approach naturally yields transferable atomic exchange, charge penetration, and dispersion parameters, facilitating the creation of versatile, optimized force fields; this approach is general, applicable to a wide array of potential applications. We then employ this approach to develop a force field, “ZIF FF”, which is tailored to accurately model CO2/N2 adsorption in zeolitic imidazolate frameworks (ZIFs). In conjunction with our previous “SYM” force field used to model adsorbate–adsorbate interactions, we compute adsorption isotherms for both CO2 and N2 in nine different ZIFs, yielding results that are in excellent accord with the corresponding experimental results. We find that ZIF FF accurately predicts isotherms for three different topologies of ZIFs (RHO, SOD, GME) and reproduces gas adsorption trends for varying functionalization across an isoreticular series of ZIFs of the GME topology. Because ZIF FF is free of empirical parameters, it presents the opportunity for computationally screening novel ZIFs that have not yet been synthesized and/or characterized.

We develop ab initio force fields for alkylimidazolium-based ionic liquids (ILs) that predict the density, heats of vaporization, diffusion, and conductivity that are in semiquantitative agreement with experimental data. These predictions are useful in light of the scarcity of and sometimes inconsistency in experimental heats of vaporization and diffusion coefficients. We illuminate physical trends in the liquid cohesive energy with cation chain length and anion. These trends are different than those based on the experimental heats of vaporization. Molecular dynamics prediction of the room temperature dynamics of such ILs is more difficult than is generally realized in the literature due to large statistical uncertainties and sensitivity to subtle force field details. We believe that our developed force fields will be useful for correctly determining the physics responsible for the structure/property relationships in neat ILs.

Molecular simulations have had a transformative impact on chemists' understanding of the structure and dynamics of molecular systems. Simulations can both explain and predict chemical phenomena, and they provide a unique bridge between the microscopic and macroscopic regimes. The input for such simulations is the intermolecular interactions, which then determine the forces on the constituent atoms and therefore the time evolution and equilibrium properties of the system. However, in practice, accuracy and reliability are often limited by the fidelity of the description of those very same interactions, most typically embodied approximately in mathematical form in what are known as force fields. Force fields most often utilize conceptually simple functional forms that have been parametrized to reproduce existing experimental gas phase or bulk data. Yet, reliance on empirical parametrization can sometimes introduce limitations with respect to novel chemical systems or uncontrolled errors when moving to temperatures, pressures, or environments that differ from those for which they were developed. Alternatively, it is possible to develop force fields entirely from first principles, using accurate electronic structure calculations to determine the intermolecular interactions. This introduces a new set of challenges, including the transferability of the resulting force field to related chemical systems. In response, we recently developed an alternative approach to develop force fields entirely from first-principles electronic structure calculations based on intermolecular perturbation theory. Making use of an energy decomposition analysis ensures, by construction, that the resulting force fields contain the correct balance of the various components of intermolecular interaction (exchange repulsion, electrostatics, induction, and dispersion), each treated by a functional form that reflects the underlying physics. We therefore refer to the resulting force fields as physically motivated. We find that these physically motivated force fields exhibit both high accuracy and transferability, with the latter deriving from the universality of the fundamental physical laws governing intermolecular interactions. This basic methodology has been applied to a diverse set of systems, ranging from simple liquids to nanoporous metal-organic framework materials. A key conclusion is that, in many cases, it is feasible to account for nearly all of the relevant physics of intermolecular interactions within the context of the force field. In such cases, the structural, thermodynamic, and dynamic properties of the system become naturally emergent, even in the absence of explicit parameterization to bulk properties. We also find that, quite generally, the three-body contributions to the dispersion and exchange energies in bulk liquids are crucial for quantitative accuracy in a first-principles force field, although these contributions are almost universally neglected in existing empirical force fields.