Effect of Higher Excited Configurations on the Linear and Nonlinear Optical Properties of Organic Molecules

Using the INDO/1 Hamiltonian and a set of configuration interaction (CI) calculations the order of which ranges from singly excited (SCI) to full CI, we demonstrate the importance of higher excited configurations on the linear and nonlinear optical (NLO) properties of two archetypical organic π-conjugated systems. The inclusion of higher excited configurations affects, both qualitatively and quantitatively, the electronic spectrum and the NLO response properties of these systems. It is shown that configurations at least at the level of singles and doubles are essential in obtaining reliable electronic and NLO properties of these chromophores. However, as the standard INDO/1 parametrization is based on a SCI level, the parameters used in higher order CI must be modified to obtain reliable properties. In this study we have found that by using a reduced value of the two-electron repulsion integral one can obtain reliable values of the linear and NLO response properties.