Ab Initio Evaluation of Henry Coefficients Using Importance Sampling

Abstract

We present a new algorithm that allows for an efficient evaluation of the Henry coefficient of a guest molecule inside a porous material, which permits to use ab initio energy calculations. The Widom insertion method, which is currently used to compute these Henry coefficients, typically requires millions of energy evaluations. Our new methodology reduces this number by more than 1 order of magnitude, enabling the use of an ab initio potential energy surface. The methodology we propose is reminiscent of the well-known importance sampling technique which is frequently used in Monte Carlo integrations. First, a conventional Widom insertion simulation is performed using a force field. In the second step, the Widom results are used to select a limited number of configurations and only for these configurations the ab initio evaluation of the energy is required. Finally, by appropriately reweighting the latter energies, an accurate estimation of the ab initio Henry coefficient is possible at a moderate computational cost. We apply our methodology to the adsorption of CO2 in Mg-MOF-74, a prototypical case where interactions of a polar guest molecule with unsaturated metal sites dominate the adsorption mechanism. In this case generic force fields such as UFF or Dreiding are inappropriate and the use of ab initio methods is indispensable. In a second case study, we compute Henry coefficients of methane in UiO-66 using different levels of theory. We pay particular attention to the influence of the dispersion corrections and the role of many-body effects. For the final example, we qualitatively investigate adsorption features for a series of functionalized UiO-66 frameworks. Overall the cases we present show that accurate computations of Henry coefficients is extremely challenging, as different levels of theory provide strongly varying results. At the same time ab initio calculations have added value compared to force fields, as they provide a physically more sound description of the adsorption mechanism and in some cases clearly improve correspondence with experiment.

Accurate transferable polarization model derived from the monomer electron density

“…76,77 A fully converged ab-initio calculation of the adsorption energy at infinite dilution, given by E ads = ∆U e −β∆U ds e −β∆U ds (16) with ∆U the insertion energy and β = 1 k B T , would require on the order of 10 6 insertions, making it computationally extremely demanding and only feasible for small and highly symmetric unit cells. 81 Therefore, we calculated insertion energies on a grid with a density of 16 Å−2 in the yz-plane through the center of the ZIF-8 unit cell. A fixed y-aligned orientation of the adsorbates was chosen, as a rotational scan at each point is too computationally demanding.…”

Analisis Molekular dan Karakteristik Hidrogen Sianida (HCN)

“…143,1 kj/mol Tabel 3. Sifat Termodinamika Hidrogen Sianida pembuatan asam sianida (HCN) yang paling sering dilakukan menggunakan metode, yang diciptakan oleh Leonid Andrussow, dimana metana (155)(156)(157)(158)(159)(160)(161)(162)(163)(164)(165)(166)(167)(168)(169)(170)(171)(172)(173)(174) , dan amonia bereaksi pada suhu [206] sekitar 1200 o C menggunakan katalis platina (175)(176)(177)(178)(179)(180)(181)(182)(183)(184)(185)(186)(187)(188)(189)(190)(191)(192)(193)(194) , dengan reaksi sebagai berikut :…”

Impact of Loading-Dependent Intrinsic Framework Flexibility on Adsorption in UiO-66

“…This comparison between the experiments and simulations is only useful if the simulations are sufficiently accurate. Modeling of adsorption in MOFs and other porous materials often assumes that the framework structure is rigid. − Assuming that the adsorbent is rigid dramatically increases the computational efficiency and eliminates the need for complicated flexible force fields to describe the frameworks. Framework flexibility in MOFs may be broadly categorized in two classes: (1) MOFs that do not significantly change the shape or volume of the unit cell in response to adsorption (Δ V = 0) and (2) MOFs that undergo significant loading-dependent structural and volumetric changes (Δ V ≠ 0) due to breathing or transition between bistable states or swelling and sub-network displacement where there is a change in unit cell volume.…”