Evaluating charge equilibration methods to generate electrostatic fields in nanoporous materials

D. Ongari, P. G. Boyd, O. Kadioglu, A. K. Mace, S. Keskin, and B. Smit, Evaluating charge equilibration methods to generate electrostatic fields in nanoporous materials J. Chem. Theory Comput. 15 (1), 382 (2019) http://dx.doi.org/10.1021/acs.jctc.8b00669

Abstract: Charge equilibration (Qeq) methods estimate the electrostatic potential of molecules and periodic frameworks by assigning point charges to each atom, using only a small fraction of the resources needed to compute density functional (DFT)-derived charges. This makes possible the computational screening of thousands of microporous structures to assess their performance for the adsorption of polar molecules. Recently, different variants of the original Qeq scheme were proposed to improve the quality of the computed point charges. One focus of this research was improving the gas adsorption predictions in Metal Organic Frameworks (MOFs), for which many different structures are available. In this work, we review the evolution of the original Qeq scheme, understanding the role of these modifications on the final output. We evaluated the result of combining different variant and set of parameters, by comparing the Qeq charges with high quality DFT-derived DDEC charges for 2338 MOF structures. We focused on the systematic errors that are attributable to specific atom types to quantify the final precision that one can expect from Qeq methods in the context of gas adsorption where the electrostatic potential plays a significant role, namely CO2 and H2S adsorption. In conclusion, both the type of algorithm and the input parameters have a large impact on the resulting charges and we draw some guidelines to help the user to choose the proper combination of the two for obtaining a meaningful set of charges. We show that, considering this set of MOFs, the accuracy of the original Qeq scheme is often still comparable with the most recent variants, even if it clearly fails in the presence of certain atom types, such as alkali metals.

© Berend Smit 2019