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.