title: Extracting electron transfer coupling elements from constrained density functional theory.
abstract: Constrained density functional theory (DFT) is a useful tool for studying electron transfer (ET) reactions. It can straightforwardly construct the charge-localized diabatic states and give a direct measure of the inner-sphere reorganization energy. In this work, a method is presented for calculating the electronic coupling matrix element (Hab) based on constrained DFT. This method completely avoids the use of ground-state DFT energies because they are known to irrationally predict fractional electron transfer in many cases. Instead it makes use of the constrained DFT energies and the Kohn-Sham wave functions for the diabatic states in a careful way. Test calculations on the Zn2+ and the benzene-Cl atom systems show that the new prescription yields reasonable agreement with the standard generalized Mulliken-Hush method. We then proceed to produce the diabatic and adiabatic potential energy curves along the reaction pathway for intervalence ET in the tetrathiafulvalene-diquinone (Q-TTF-Q) anion. While the unconstrained DFT curve has no reaction barrier and gives Hab approximately 17 kcal/mol, which qualitatively disagrees with experimental results, the Hab calculated from constrained DFT is about 3 kcalmol and the generated ground state has a barrier height of 1.70 kcal/mol, successfully predicting (Q-TTF-Q)- to be a class II mixed-valence compound.
authors.affiliation: Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. qinwu@mit.edu
authors.forename: Qin
authors.initials: Q
authors.lastname: Wu
authors.forename: Troy
authors.initials: T
authors.lastname: Van Voorhis
mesh_terms: D004583:Electrons; D006571:Heterocyclic Compounds / Q000737:chemistry; D015394:Molecular Structure; D011809:Quinones / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D006571:Heterocyclic Compounds; D011809:Quinones; C063887:tetrathiafulvalene