MSINDO-sCIS: A New Method for the Calculation of Excited States of Large Molecules

Abstract

Theoretical background, parametrization, and performance of the semiempirical configuration interaction singles (CIS) method MSINDO-sCIS designed for the calculation of optical spectra of large organic molecules are presented. The CIS Hamiltonian is modified by scaling of the Coulomb and exchange integrals and a semiempirical correction. For a recently proposed benchmark set of 28 medium-sized organic molecules, vertical excitation energies for singlet and triplet states are calculated and statistically evaluated. A full reparameterization of the MSINDO method for both ground and excited state properties was necessary. The results of the reparameterized MSINDO-sCIS method are compared to the currently best semiempirical method for excited states, OM3-CISDTQ, and to other standard methods, such as MNDO and INDO/S. The mean absolute deviation with respect to the theoretical best estimates (TBEs) for MSINDO-sCIS is 0.44 eV, comparable to the OM3 method but significantly smaller than for INDO/S. The computational effort is strongly reduced compared to OM3-CISDTQ and OM3-MRCISD, since only single excitations are taken into account. Higher excitations are implicitly included by parametrization and an empirical correction term. By application of the Davidson–Liu block diagonalization method, high computational efficiency is achieved. Furthermore, it is demonstrated that the MSINDO-sCIS method correctly describes charge-transfer (CT) states that represent a problem for time-dependent density functional theory (TD-DFT) methods