The structures of the hydrogen-bonded complexes of nitric acid with various bases: CH4... H-ONO2, N-2... H-ONO2, OC ... H-ONO2, NO2... H-ONO2, (HNO3)(2) and H3N ... H-ONO2 are studied extensively by ab initio calculations at SCF and MP2 levels with various basis sets: 6-31G/SCF, 6-31C/MP2, 6-31G**/SCF, 6-31G**/MP2, 6-31+G**/SCF, 6-31+G**/MP2, 6-311+G**/SCF and 6-311+G**/MP2 and density functional B3LYP/6-31G(d,p) calculations. Full geometry optimization was made for the complexes studied. After the consideration of the strength of the hydrogen bond, the studied hydrogen-bonded complexes can be order as follows: H3N .. H-ONO2, (HNO3)(2), NO2H-ONO2, OC ... H-ONO2, N-2... H-ONO2, CH4... H-ONO2. (C) 2000 Elsevier Science B.V. All rights reserved.