Abstract
Density functional theory (DFT) is widely applied in calculations of molecules and materials. Yet, it suffers from a well-known over-emphasis on charge delocalization arising from self-interaction error that destabilizes localized states. Here, using the symmetric diamine N,N′-dimethylpiperazine as a model, we have experimentally determined the relative energy of a state with positive charge localized on one of the two nitrogen atoms, and a state with positive charge delocalized over both nitrogen atoms. The charge-localized state was found to be 0.33 (0.04) eV higher in energy than the charge-delocalized state. This provides an important test of theoretical approaches to electronic structure calculations. Calculations with all DFT functionals commonly used today, including hybrid functionals with exact exchange, fail to predict a stable charge-localized state. However, the application of an explicit self-interaction correction to a semi-local functional identifies both states and gives relative energy in excellent agreement with both experiment and CCSD(T) calculations.
Highlights
Density functional theory (DFT) is widely applied in calculations of molecules and materials
The commonly used Kohn–Sham density functional theory (DFT) functionals are known to suffer from bias towards delocalized states because of self-interaction error[13,14,15]
We show here that all the commonly used DFT functionals, including hybrid functionals, fail to produce the localized charge state of the system studied, calculations using PZ-SIC give results in excellent agreement with the experimentally determined relative energy of a localized and delocalized electronic state
Summary
Density functional theory (DFT) is widely applied in calculations of molecules and materials. Over 30 years ago, an explicit self-interaction correction approach was proposed by Perdew and Zunger (PZ-SIC)[16] It was applied early on in studies of the electronic structure of molecules We show here that all the commonly used DFT functionals, including hybrid functionals, fail to produce the localized charge state of the system studied, calculations using PZ-SIC give results in excellent agreement with the experimentally determined relative energy of a localized and delocalized electronic state. To achieve this comparison, experimental benchmarks for chargelocalized and charge-delocalized states are required. We aim to evaluate the capability of each method to properly describe the charge localization and delocalization in the ground electronic state of the molecular cation
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