We have measured the polarized Raman cross sections and depolarization ratios of 16 fundamental modes of nickel octaethyltetraphenylporphyrin in a CS2 solution for 16 fundamental modes, i.e., the A(1g)-type vibrations nu(1), nu(2), nu(3), nu(4), nu(5), and phi(8), the B-1g vibrations nu(11) and nu(14), the B-2g vibrations nu(28), nu(29), and nu(30) and the antisymmetric A(2g) modes nu(19), nu(20), nu(22), and nu(23) function of the excitation wavelength. The data cover the entire resonant regions of the Q- and B-bands. They were analyzed by use of a theory which describes intra- and intermolecular coupling in terms of a time-independent nonadiabatic perturbation theory [E. Unger, U. Bobinger, W. Dreybrodt, and R. Schweitzer-Stenner, J. Phys. Chem. 97, 9956 (1993)]. This approach explicitly accounts in a self-consistent way for multimode mixing with all Raman modes investigated. The vibronic coupling parameters obtained from this procedure were then used to successfully fit the vibronic side bands of the absorption spectrum and to calculate the resonance excitation profiles in absolute units. Our results show that the porphyrin macrocycle is subject to B-2u-(saddling) and B-1u-(ruffling) distortions which lower its symmetry to S-4 Thus, evidence is provided that the porphyrin molecule maintains the nonplanar structure of its crystal phase in an organic solvent. The vibronic coupling parameters indicate a breakdown of the four-orbital model. This notion is corroborated by (ZINDO/S) calculations which reveal that significant configurational interaction occurs between the electronic transitions into Q]- and 1B]-states and various porphyrin-->porphyrin, metal-->porphyrin, and porphyrin-->metal transitions. The intrastate coupling parameters are used to estimate the excited electronic states’ displacements along the normal coordinates with respect to the ground state and their contributions to the reorganization energy, It turns out that the B]-state is predominantly affected by symmetric A(1g)-displacements, whereas the Q]-state is subject to A(2g), B-1g, and B-2g displacements of its equilibrium configuration. While the former is induced by the combined effect of ruffling and saddling, the latter arises from Jahn-Teller coupling within the degenerate states.