The spectroscopic properties of a molecule is strongly dependent on interactions with the surrounding environment. We have used time dependent density functional theory (TDDFT) and a multilevel perturbation model to study the transition energies, polarizability, and dipole moments of a bacteriochlorophyll in an electric field. The perturbation Hamiltonian was constructed as to give an electric field dependence of the transition energies in agreement with the result from TDDFT. It was found that, to reach agreement, it was not enough to use the Q and Soret bands of the bacteriochlorophyll; additional energy levels had to be introduced. The change in dipole moment and polarizability for the two lowest excited states Q(x) and Q(y) were calculated. Our calculations predict a negative change in polarizability Deltaalpha for the ground state to Q(y) transition.
