An effective model for the X 2A1-A 2B2 conical intersection in NO2

We propose an efficient method for calculating the eigenstates and adjusting the parameters of an effective Hamiltonian, which reproduces the experimentally observed energy levels of NO2 up to 11 800 cm(-1) above the quantum mechanical ground state, that is a few thousands of cm(-1) above the X (2)A(1)-A B-2(2) conical intersection, with a rms error less than 4 cm(-1). This method principally relies on the determination, through first-order perturbation theory, of an optimal basis for each surface, which takes into account the nonresonant energy shifts experienced by the states of this surface. As a result, the size of the matrix, which one has to build and diagonalize to converge the spectrum up to 11 800 cm(-1), is of the order of 500-1000 instead of several tens of thousands. Thank to this Hamiltonian, the analysis of the experimental spectrum up to 11 800 cm(-1) could be completed. A detailed description of all states located above 9500 cm(-1) is proposed, those lying below 9500 cm(-1) being already known and tabulated. (C) 2003 American Institute of Physics.