The S-0 (X(1)A'), T-1 (a(3)A"), S-1 (A(1)A"), T-2 (b(3)A") and S-2 (B(1)A') states of the (trans-)HONO molecule were studied by using the CASSCF and CASPT2 methods. The CASPT2(//CASPT2) adiabatic and vertical excitation energy values are in good agreement with available experimental data. The CASPT2//CASSCF potential energy curves (PECs) calculations indicate that: (i) all the five states correlate with the products of OH (X-2 Pi) + NO (X-2 Pi); (ii) along each of the T-1, S-1, and T-2 PECs there is a minimum followed by a transition state (barrier); and (iii) the repulsive S-2 PEC crosses the T-2, S-1, and T-1 PECs. The geometries and relative energies for the stationary points along these PECs were calculated at the CASPT2(//CASPT2) level, and the calculations predict that the barrier height value for S-1 is negligibly small (0.0018 eV). (C) 2007 Elsevier B.V. All rights reserved.