The potential-energy surfaces of the 1 (1)A(g), 2 (1)A(g), and 1 B-1(u) sta
tes of trans-1,3,5-hexatriene (THT) are explored in the vicinity of the gro
und state equilibrium structure. The S-0 geometry optimization and force fi
eld calculation have been carried out with the restricted Hartree-Fock plus
Moller-Plesset second-order perturbation theory method. Vibronic coupling
constants for the normal coordinates of a(g) and b(u) symmetry were compute
d with the complete-active-space self-consistent-field (CASSCF) and single
state multiconfigurational second-order perturbation theory (CASPT2) electr
onic structure models. The CASSCF/CASPT2 method unequivocally places the ve
rtical excitation energy of the dark 2 (1)A(g) "phantom state" below the 1
B-1(u) level and predicts an energy difference of ca. 0.5 eV. The results a
re consistent with time-resolved photoionization yield and photoelectron sp
ectroscopy experiments that indicate the existence of a low lying S-1-S-2 c
onical intersection which induces rapid 1 B-1(u)--> 2 (1)A(g) internal conv
ersion on a time scale of 40 fs to 50 fs [Cyr and Hayden, J. Chem. Phys. 10
4, 771 (1996)]. Based on the vibronic coupling constants five totally symme
tric vibrations with high Franck-Condon and/or tuning activity have been id
entified. The S-1 and S-2 states interact primarily via the two b(u) normal
modes nu(24) and nu(26). Other a(g) and b(u) normal vibrations do not appe
ar to couple significantly to the lowest lying pi -->pi(*) transition. The
modeling of the ultrafast relaxation processes following optical excitation
of the 1 B-1(u) state of THT and the calculation of absorption and resonan
ce Raman spectra are discussed in the following paper. (C) 2000 American In
stitute of Physics. [S0021-9606(00)30502-5].