Fast and Accurate Calculation of the UV–Vis Spectrum with the Modified Local Excitation Approximation

The local excitation approximation (LEA), a method for
the calculation
of electronic excitations localized in a specific region of a molecule,
has been modified with new approaches to enhance the accuracy of the
original method. The primary concept behind LEA involves isolating
the region of interest as a submolecule from the full molecule using
a localization method, followed by calculating electronic excitations
solely within this submolecule. In this study, we examined approaches
that improve the accuracy in describing the region of interest, particularly
its molecular orbital energies. Additionally, the localization method
was extended with a new projection technique to accelerate calculations.
These approaches were studied in time-dependent density functional
theory (TDDFT) calculations applied to four testing systems with a
chromophore as the region of interest: two basic linear molecules,
acrolein surrounded by 24 water molecules, and a model of a green
fluorescent protein. For all studied systems, the results of TDDFT
calculations combined with LEA exhibited near-zero error when groups
of atoms adjacent to the chromophore were explicitly included in the
submolecule. This was achieved with at least a quadratic speedup of
the calculation time as a function of the submolecule size.