Ab initio calculation of UV-Vis absorption spectra of a single molecule chlorophyll a: Comparison study between RHF/CIS, TDDFT, and semi-empirical methods

摘要

Chlorophyll a is one the most abundant pigment on Earth that responsible fortrapping the light energy to perform photosynthesis in green plants. Thismolecule is a metal-complex that consists of a porphyrin ring with highsymmetry that acts as ligands with magnesium as the central ion. Chlorophyll ahas been studied for many years from different point of views for bothexperimental and theoretical interests. In this study, the restrictedHartree-Fock configuration interaction single (RHF/CIS), time-dependent densityfunctional theory (TDDFT) and some semi-empirical methods (CNDO/s and ZINDO)calculations were carried out and compared to reconstruct the UV-Vis absorptionspectra of chlorophyll a. In some extend, the calculation results based on asingle molecule calculation were succeeded to reconstruct the absorptionspectra but required to be scaling to match the experimental one. Differentcomputational methods (ab initio and semi-empirical) exhibit the differences inthe energy correction factor and the presence of transition states, however,still conserve the main spectral features. In general, the semi-empiricalmethods provide a better energy scaling factor, which means closer to theexperimental one. However, they lack of fine features or vertical transitionswith respect to the experimental spectra. The ab initio calculations resultmore complete features than the semi-empirical methods, especially the TDDFTwith high level of basis sets that provides a good accuracy in transitionenergies. The contribution of ground states and excited states orbitals in themain vertical transitions is discussed based on orbital structure. This mightgives a new perspective to explain the energy transfer phenomena in theabsorption processes that related to the function of chlorophyll a as alight-harvesting antenna.