Absorption-energy calculations of chlorophyll a and b with an explicit solvent model

Abstract

The absorption energies of the Qy bands of chlorophylls a and b (Chla and Chlb) in organic solvents (acetone, diethyl ether, and ethanol) were calculated by combining molecular dynamics simulations and quantum mechanical/molecular mechanical approaches with an explicit solvent model. It was found that excitation-energy calculations by time-dependent density functional theory (DFT) using the CAM-B3LYP functional, following DFT geometry optimizations with the B3LYP functional, accurately reproduced the differences between the observed absorption energies of Chla/Chlb in each solvent. The calculated energies were within a root-mean-square deviation of 0.0014 eV from the observed values when the CAM-B3LYP-related parameter μ, which is associated with the long-range correlation, was set to 0.14. Calculations using μ = 0.14 also reproduced the observed transition-dipole strengths of the Qy-band moments, and the observed absorption spectra, with linewidths of ∼0.05 eV. These results suggest that μ = 0.14 can be used for absorption-energy calculations for Chla and Chlb.