The existence of the peroxo nitrate radical (ONOO) has been discussed for some time and its formation has been used to explain aspects of the NO3 scavenging in the Earth's atmosphere. In this study we report our thorough investigation of the stability of this species by means of highly correlated ab initio calculations. Single-reference coupled-cluster singles and doubles (CCSD) as well as multireference configuration interaction (MRCI) calculations were performed to optimize equilibrium structures and obtain harmonic force fields. The force fields were used to calculate the harmonic frequencies as well as isotopic shifts. The CCSD calculations result in shallow minima for both the \(2)A"> ground as well as the \(2)A'> excited state. However, the calculated isotopic shifts of the ground state show that the experimentally observed shift of 50 cm(-1) cannot be due to ONOO. In contrast, no minima were found by the MRCI calculations. The analysis of the wave functions indicates that the potential wells obtained by CCSD are artifacts which are due to the single-reference nature of the CCSD method. Our conclusion is that ONOO is not a bound structure and cannot be observed experimentally. Our calculations also show that thermal decomposition of NO3 into NO and O-2 is not possible under atmospheric conditions and thus this channel cannot be responsible for the unknown NO3 scavenging process discussed in the literature. (C) 2003 American Institute of Physics.