Semiclassical dynamics simulation of the excitation of one of two pi-stacked adenines by a 25 fs fwhm laser pulse was found to lead to the formation of a long-lived excimer, with an average C-2-C'(2) distance of 2.2 angstrom. The C'(2)-N'(3) bond in the unexcited adenine was found to twist after the formation of excimer. The out-of-plane deformations of both C-2 and C'(2) atoms play a significant role in the vibronic coupling, between the HOMO and LUMO, which leads to the deactivation of the excimer. This interaction is attributed to a face-to-face stacked configuration and a very short C-2-C'(2) distance of 1.821 angstrom. (C) 2011 Elsevier B. V. All rights reserved.