We introduce an 'on-the-fly' diabatization in the context of ab initio multiple spawning simulations. Adiabatic potential energy surfaces are computed and diabatized by using the block-diagonalization method simultaneously with nuclear wavepacket dynamics. Because the diabatization is carried out without knowledge of the nonadiabatic coupling matrix elements, we are able to use the electronic structure methods where those matrix elements are difficult to calculate, such as equation-of-motion coupled-cluster singles and doubles theory. The electronic absorption spectra of the lowest Rydberg state of ethylene is studied by applying our new approach, and we elucidate the origin of the doublet feature in the spectrum. (C) 2004 Elsevier B.V. All rights reserved.