Efficient Implementation of Local Excitation Approximation for Treating Excited States of Molecules in Condensed Phase.

Abstract

An efficient implementation of the local excitation approximation (LEA) of time-dependent density functional theory (TDDFT) or time-dependent Hartree-Fock, (TDHF) (or configuration interaction singles, CIS) method has been developed in this work. The LEA-TDDFT, -TDHF, and -CIS methods have been applied to investigate the solvatochromic shift of the n → π* vertical excitation energy of acetone in aqueous solution. The main idea of the LEA scheme is that only local electron excitations within a certain active region (called as chromophore) are treated to obtain the excitation energies for locally excited electronic states. We have proposed an efficient localization procedure to obtain regional localized molecular orbitals (RLMOs) localized on the chromophore subunit. To ensure the accuracy of the TDDFT, TDHF, and CIS schemes for the studied system, we choose one acetone and six nearest-neighboring waters as the active region for each acetone-water cluster. For acetone in aqueous solution, the LEA-TDDFT calculations on 600 acetone-water configurations (generated from molecular dynamics simulation) suggest that the blueshift in the n → π* vertical electronic excitation energy is 1621 ± 52 cm(-1), which is in good agreement with the available experimental blue shift of 1500-1700 cm(-1).