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

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.