Chromophores in spectroscopy: Ab initio studies of localized descriptions of molecular electronic excitations

We review a computationally efficient approach, based jointly on the Random Phase Approximation (RPA) and on localized molecular orbitals, for calculating and analyzing electronic excitations in terms of the nature of the chromophore and its interaction with its molecular surroundings. The method is applied to two typical chromophoric systems using ab initio extended-basis calculations: the non-conjugated but electronically coupled ethylenic double bonds in norbornadiene (NBD, bicyclo[2.2.1]hepta-2,5-diene) and the chirally perturbed carbonyl chromophore inequatorial 4-methyladamantanone (EMAO). The analyses are a posteriori in nature but provide insights into the spectroscopic properties of medium-sized molecules.