The estimates for the vertical excitation energy of the 2 (1)A(1) state of cis-1,3,5-hexatriene (CHT) vary considerably and provide a good example of the difficulties that can arise in determining transition energies. The great uncertainty is surprising if one considers that this state has already been characterized by high resolution techniques such as resonance enhanced multiphoton ionization (REMPI) and fluorescence excitation spectroscopy in free jet expansions. A theoretical analysis of this problem is clearly needed and the present work, along with the following paper, represents an effort to investigate the nature of the 2 (1)A(1) and 1 B-1(1) states of CHT. It is shown that a combination of nb initio electronic structure and quantum-mechanical wave packet calculations is required to systematically approach a question as involved as locating the energetical position of the 2 (1)A(1), level. We characterize the energy dependence of the 1 (1)A(1), 2 (1)A(1), and 1 B-1(1) states of CHT as a function of the in-plane normal coordinates for small displacements from the ground-state equilibrium geometry employing the single-state multiconfigurational second-order perturbation theory (CASPT2) method. This information constitutes the basis for the construction of diabatic harmonic model potential-energy functions associated with the three electronic states in the Franck-Condon region that is essential for the treatment of nonadiabatic dynamics. Five totally symmetric modes with high Franck-Condon and/or tuning activity are identified. Vibronic interaction between the S-1 and S-2 states is primarily mediated by four vibrations of b(1) symmetry, nu (26), nu (27), nu (30), and nu (31). nu (30) and nu (31) are found to be exceptionally powerful interstate coupling modes and the strong nonadiabatic effects induced by these modes in CHT are mainly responsible for the spectroscopic differences observed for the S-1 and S-2 states of CHT and trans-1,3,5-hexatriene. (C) 2001 American Institute of Physics.