An approximate ab initio sum-over-states approach to the calculation of non-linear optical properties in organic molecules

We present an ab initio formulation of a modified sum-over-states method to perform calculations on the non-linear optical (NLO) properties for intermediate and large molecular systems. A benchmark application to the trans-octatetraene molecule is presented with comparison to other ab initio calculations. The method is also used to compute some key NLO parameters for the molecules di-alkyl-amino-nitro-stilbene and 3,4-methoxy-2,5-bis-(benzo-thiazolyl-2)thiophene treated earlier at the semi-empirical level by the present authors.