The Dependence of Spurious Charge-Transfer Excited States on Orbital Exchange in TDDFT: Large Molecules and Clusters

Published

April 20, 2007

Author(s)

Rudolph Magyar, S Tretiak

Abstract

Time-dependent density functional theory (TDDFT) is a powerful tool allowing for accurate description of excited states in many nano-scale molecular systems; however, its application to large molecules may be plagued with difficulties that are not immediately obvious from previous experiences of applying TDDFT to small molecules. In TDDFT, the appearance of spurious charge-transfer states below the first optical excited-state is shown to have significant effects on the predicted absorption and emission spectra of several donor-acceptor substituted molecules. The same problem affects the predictions of electronic spectra of molecular aggregates formed from weakly interacting chromophores. For select benchmark cases, we show that today s popular density functionals, such as purely local (the local density approximation, LDA) and semi-local (generalized gradient approximation, GGA) models, are not just quantitatively inaccurate but are qualitatively wrong. Non-local hybrid approximations including both semi-empirical (B3LYP) and ab initio (PBE1PBE), contain a small fraction (20-25%) of Fock-like orbital exchange butare also susceptible to such problems. Functionals that contain a larger fraction (50%) of orbital exchange like the early hybrid (BHandH) are shown to exhibit far fewer spurious charge transfer (CT) states. This leads us to conclude that accurate practical functionals must at least mimic or explicitly contain orbital exchange to reliably describe excitations in nano-systems.

Citation

Journal of Chemical Theory and Computation

Volume

3

Issue

3

Pub Type

Journals

Keywords

charge transfer, density functional theory, nano-systems

Citation

Magyar, R. and Tretiak, S. (2007), The Dependence of Spurious Charge-Transfer Excited States on Orbital Exchange in TDDFT: Large Molecules and Clusters, Journal of Chemical Theory and Computation (Accessed April 30, 2024)