The vibrational spectrum of nitrous acid (HONO) in the electronic ground state is derived from fully coupled quantum-mechanical calculations. A global six-dimensional potential hypersurface is directly interpolated from density-functional calculations. A potential-optimized generalized coordinate discrete variable representation is combined with multidimensional successive truncation to yield the complete vibrational spectrum up to the first OH stretching overtone of both stable isomers. Higher OH stretching overtones up to 7nu(OH) are calculated selectively applying the adiabatic contraction technique. The ab initio prediction of the vibrational spectrum is in remarkably good agreement with available experimental data. The results indicate the onset of the intramolecular (1,3)-H-transfer between the first and second OH stretching overtone of cis-HONO. (C) 2003 American Institute of Physics.