An ab initio computational study was performed to study the effect of hydrogen bonding in the form of base pairings on the ground and lowest singlet pi pi* excited state geometries of the guanine unit. The studied systems consist of isolated guanine, guanine-cytosine and guanine-guanine base pairs in two different H-bonding configurations (GG16 and GG17). The ground state geometries were optimized at the Hartree-Fock level, while the lowest singlet pi pi* electronic excited state geometries were optimized at the CIS level. The 6311G(d,p) basis set was used in all calculations. The nature of the potential energy surfaces was ascertained via harmonic vibrational frequency analysis; all structures were found to be minima. The origination of electronic singlet pi pi* excitation of base pairs is attributed to the excitation of the guanine monomer, except for the GG16 base pair for which the whole complex was excited. The ring geometry of guanine in the isolated, GC, and GG17 base pairs in the excited state was found to be strongly nonplanar, while in the GG16 base pair it was found to be almost planar. (c) 2005 Elsevier B.V. All rights reserved.