Density functional theory (DFT) is the most widely-used electronic structure approximation across chemistry, physics, and materials science. Every year, thousands of papers report hybrid DFT simulations of chemical structures, mechanisms, and spectra. Unfortunately, hybrid DFT's accuracy is ultimately limited by tradeoffs between over-delocalization and under-binding. This review summarizes these tradeoffs, and introduces six modern attempts to go beyond them while maintaining hybrid DFT's relatively low computational cost: DFT+U, self-interaction corrections, localized orbital scaling corrections, local hybrid functionals, real-space nondynamical correlation, and our rung-3.5 approach. The review concludes with practical suggestions for DFT users to identify and mitigate these tradeoffs' impact on their simulations.

Dissolution of lignocellulose in ionic liquids is a promising route to synthesizing fuels and chemical feedstocks from woody plant materials. While cellulose dissolution is well-understood, less is known about the differences between ionic liquids' interactions with cellulose vs.lignin. This work uses dispersion-corrected density functional theory (DFT-D) to model the interactions of imidazolium chloride ionic liquid anions and cations with (1,4)-dimethoxy-β-D-glucopyranose and 1-(4-methoxyphenyl)-2-methoxyethanol as models for cellulose and the lignin polyphenol, respectively. The cellulose model preferentially interacts with Cl⁻, confirming previous experimental and theoretical studies. However, the lignin model has significant π-stacking and hydrogen bonding interactions with imidazolium cation. These results are robust to changes in the computational details, and suggest that the ionic liquid cations play important roles in tuning the relative solubilities of lignin and cellulose. Calculations predict that the extended π-systems of benzimidazolium ionic liquids yield stronger interactions with lignin, showing potential for improved lignocellulose solvents.

This work presents a computational mechanistic study of the acid-catalyzed hydrolysis of lignin β-O-4 linkages in ionic liquid solvents. Model compound 2-hydroyxyethyl phenyl ether undergoes dehydration to vinyl phenyl ether followed by hydrolysis to phenol and “Hibbert's ketones”. Larger model compound α-hydroxy-phenethyl phenyl ether illustrates an E1 dehydration mechanism involving resonance-stabilized carbocations. Continuum models for ionic liquid solvents indicate that solvation can significantly affect the reaction rates. The tested continuum ionic liquid solvents give similar results, and differ significantly from continuum organic solvents with comparable dielectric constants. The acidic ionic liquid cation 1-H-3-methylimidazolium has lower predicted catalytic activity than hydronium or HCl, consistent with the former's relatively small acid dissociation constant. Calculations with dispersion-corrected density functionals give similar behavior. Calculations on Lewis acidic metal chlorides used experimentally for lignin hydrolysis suggest that the metal chloride may participate in the initial dehydration. These results provide a baseline for future studies of improved hydrolysis catalysts.

This work proposes a new term on the “Jacob’s ladder” of approximate exchange-correlation functionals in Kohn–Sham density functional theory. In these Rung 3.5 functionals, the exchange-correlation energy density at a point depends linearly (rather than quadratically) on the nonlocal Kohn–Sham one-particle density matrix in a finite neighborhood around the point. These functionals are intermediate in complexity between the semilocal approximations of Rungs 1–3, and fully nonlocal Rung 4 approximations such as global hybrids. Rung 3.5 functionals built on the model for exchange in [B. G. Janesko, J. Chem. Phys. 131, 234111 (2009)] predict molecular thermochemistry and kinetics with accuracy intermediate between their “parent” semilocal functional and the corresponding global hybrid. The best Rung 3.5 functional presented here gives mean absolute errors of 5.7 kcal/mol for G3/99 thermochemistry, 5.2 kcal/mol for HTBH38/04 hydrogen-transfer reaction barriers, and 5.7 kcal/mol for NHTBH38/04 nonhydrogen-transfer reaction barriers, while incorporating only two empirical parameters.