A
Configuration Interaction Picture for a Molecular
Environment Using Localized Molecular Orbitals: The Excited States
of Retinal Proteins
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Abstract
Electronic excitations of chromophores in proteins and
solutions
are associated with the electronic response of the molecular environment.
The underlying interactions are important origins of solvatochromism.
We performed large-scale configuration interaction singles (CIS) calculations
(up to 1000 atoms) for retinal chromophores in proteins and methanol
solution, in which one-electron processes (polarization and charge-transfer
effects of the environment) are included. The present approach also
improved the electrostatic potential, as compared to that described
by a molecular mechanics (MM) force field. The CIS results were combined
with the symmetry adapted cluster (SAC)-CI result using our own N-layer
integrated molecular orbital molecular mechanics (ONIOM) method. As
compared to the MM description, the CIS reduces the calculated excitation
energy by 0.1–0.3 eV and also improves the relative excitation
energies among retinal proteins. We applied our localized molecular
orbital (LMO) transformation scheme to analyze the CI wave functions.
The result clarified the contributions of the amino acids. In bacteriorhodopsin,
Tyr185 contributes intermolecular CT excitations. The radial distribution
of amino acids’ contributions to the CI wave function was also
analyzed. The results of the analysis are useful not only for understanding
the molecular interactions and the role of amino acids in color tuning,
but also for providing insight into the structure of the excited-state
wave function for the molecular environment. An excitation-energy
decomposition analysis also supported the results of the excited-state
wave functions