Vibrational frequency analysis for the Watson-Crick guanine-cytosine (GC) base pair in the ground and lowest single pi pi(*) excited state was performed based on the reference geometry optimized at the HF/6-311G(d,p) and CIS/6-311G(d,p) levels, respectively. It was found that different NH and CH stretching vibrations can be used for the qualitative prediction of excited state geometrical non-planarity and the localization of electronic excitations in the base pair complexes. (C) 2007 Elsevier B.V. All rights reserved.