How does the nonlocal HF exchange influence the electron excitation of Bacteriochlorophyll and its assembly

We perform the time-dependent long-range corrected density functional theory (TD-LRC-DFT) studies on the electron excitation of Bacteriochlorophyll (BChl) and its assembly in two peripheral light-harvesting complexes (known as LH2) from purple bacteria. It is found that LRC-DFT gives a more reasonable description to the excitation energies and the energy spacings of both the single BChl pigment and its assembly than the conventional hybrid functionals, such as B3LYP and PBE0. The long-range correction to the conventional DFT exchange functionals is essential to correctly describe their electron excitation. For a single BChl pigment, both Q(y) and Q(x) excitations are highly delocalized, and Q(y) excitation involves obvious metal-to-ligand intramolecular charge-transfer character, and thus, it is more sensitive to xc functionals and solutions. From the results of three typical BChl pairs of LH2 complex, it is found that localized intramolecular Frenkel excitons (FEs) dominate the energy transfer process. Slight mixing between FEs and intermolecular charge-transfer excitons (CTEs) is observed in the pair 1 alpha 1 beta of 850 ring. The conventional functionals, such as B3LYP, underestimate intermolecular CTE energies, lay CTEs close to both Q(y) and Q(x) states. In this case, the mixing degree between the intramolecular FEs and intermolecular CTEs increases, which induces much narrow energy spacing between two lowest excited states of the pair 1 alpha 1 beta. It is thus incorrect to use the energy splitting of 1 alpha 1 beta resulted from B3LYP as the scalar to estimate the excitonic coupling and site energies as well. (C) 2011 Elsevier B.V. All rights reserved.