We have used the MOR BID Hamiltonian and computer program (P. Jensen, J. Mol. Spectrosc. 128, 478-501. 1988; J. Chem. Soc. Faraday Trans. 2 84, 1315-1340. 1988; in "Methods in Computational Molecular Physics" (S. Wilson and G. H. F. Diercksen, Eds.), Plenum, New York, 1992 ) to refine the potential energy function for the electronic ground state (\hat{X}^{2} A_{1}) of (\mathrm{NO}{2}). The input data for the adjustment of the parameters consisted essentially of the extensive set of vibrational energies measured by A. Delon and R. Jost (J. Chem. Phys. 95, 5686-5700, 1991). The primary aim of the present work is to provide a potential energy surface which can be used to calculate rovibrational transitions for (\mathrm{NO}{2}), for example in atmospheric studies. We have neglected the vibronic interaction between the (\tilde{X}^{2} A_{1}) state and the first excited electronic state, (\tilde{A}^{2} B_{2}) (at (10000 \mathrm{~cm}^{-1}) ). Recent ab initio calculations (G. Hirsch. S. Carter, and R. J. Buenker, private communication) suggest, however, that this interaction significantly influences the vibronic energy pattern at energies as low as (5000 \mathrm{~cm}^{-1}). This would imply that in the energy region above 5000 (\mathrm{cm}^{-1}). our fitted potential energy surface is effective in the sense that its parameters have adjusted to accommodate the energy displacements due to the interaction with the (\tilde{A}^{2} B_{2}) electronic state. c. 1994 Academic Press. Inc.