Quantitative vibronic coupling calculations: the formyloxyl radical

The performance of different choices of parametrization for the quasidiabatic Hamiltonian approach of Koppel et al. (Adv Chem Phys 57:59, 1984) is studied for the states of the formyloxyl radical, . Two basic classes of parametrization are analyzed. In the first (vertical), the model Hamiltonian is constructed in such a way that it reproduces an ab initio surface near the geometry of the absorbing state in a spectroscopic transition; the second scheme (adiabatic) is designed to provide a faithful representation of the ab initio surface near the final state structures. The latter is better suited to treat energy level spacings, especially at relatively low energies, while the former may offer advantages in some cases where it may provide a more balanced description of the final states and/or features such as seams of conical intersections. For , the most interesting and prominent feature of the vibronic coupling scheme is a strong coordinate dependence of the coupling between the two electronic states. This effect renders effectively useless parametrization approaches in which the linear coupling strength is assumed constant. Moreover, through selective softening of the (2) A (1) state potential and corresponding reduction in its zero-point energy, this seemingly arcane property of the vibronic coupling in is principally responsible for the ground state of this molecule being (2) A (1) instead of (2) B (2), as the adiabatic potential energy surface minimum of the latter is some 500 lower.