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