Ab initio theory for treating local electron excitations in molecules and its performance for computing optical properties

In this article, as a first step to develop an efficient approximation for predicting the molecular electronic excited state properties at ab initio level, we propose local excitation approximation (LEA). In the LEA scheme, the only local electron excitations within selected substructure (Chromophore) are treated to calculate the targeted, excited state wavefunctions, whereas the other electron excitations (local electron excitations in other substructure and. charge-transfer excitations between different regions) are simply discarded. This concept is realized by using the localized molecular orbitais (LMO) localizing on the chromophore substructure. If the targeted transitions show the strong local character and the adequate substructure is selected as chromophore region, the LEA scheme can provide excited state properties without large loss of accuracy. The fatal slowdown of convergence speed of Davidson's iterative diagonalization due to the use of LMO can be avoided by additional transformation of LMOs. To assess the accuracy and efficiency of the LEA scheme, we performed test calculations using various compounds at configuration interaction single (CIS) and time-dependent Hartree-Fock (TDHF) level of theory.