Investigating the Density-Corrected SCAN using Water Clusters and Chemical Reaction Barrier Heights

Abstract

Kohn-Sham density functional theory (KS-DFT) is one of the most widely used electronic structure methods. It is used to find the various properties of atoms, molecules, clusters, and solids. In principle, results for these properties can be found by solving self-consistent one-electron Schrödinger-like equations based on density functionals for the energy. In practice, the density functional for the exchange-correlation contribution to the energy must be approximated. The accuracy of practical DFT depends on the choice of density functional approximation (DFA) and also on the electron density produced by the DFA. The SCAN(strongly constrained and appropriately normed) functional developed by Sun, Ruzsinszky, and Perdew is the first meta-GGA (meta-generalized gradient approximation) functional that is constrained to obey all 17 known exact constraints that a meta-GGA can. SCAN has been found to outperform most other functionals when it is applied to aqueous systems. However, density-driven errors (energy errors occurring from an inexact density produced by a DFA) hinder SCAN from achieving chemical accuracy in some systems, including water. Density-corrected DFT (DC-DFT) can alleviate this shortcoming by adopting a more accurate electron density which, in most applications, is the electron density obtained at the Hartree-Fock level of theory, due to its relatively low computational cost. In the second chapter, calculations to determine the accuracy of the HF-SCAN functional for water clusters are performed. The interaction and binding energies of water clusters in the BEGDB and WATER27 data sets are computed, and then the spurious charge transfer in deprotonated, protonated, and neutral water dimer is interpreted. The density-corrected SCAN (DC-SCAN) functional elevates the accuracy of SCAN toward the CCSD(T) limit, not only for the neutral water clusters but also for all considered hydrated ion systems (to a lesser extent). In the third chapter, the barrier heights of the BH76 test set are analyzed. Three fully non-local proxy functionals (LC-ωPBE, SCAN50%, and SCAN-FLOSIC) and their selfconsistent proxy densities are used. These functionals share two important points of similarity to the exact functional. They produce reasonably accurate self-consistent barrier heights and their self-consistent total energies are nearly piecewise linear in fractional electron number. Somewhat-reliable cancellation of density - and functional-driven errors for the energy has been established.