The calculations about the interaction between cytosine and nitrogen monoxide have been made and six isomers with minimum energies have been found. Both N and O can combine with N—H bonds of cytosine and then hydrogen bonds between them are formed. The nitrogen atom of NO has a privilege to bind with cytosine, which makes nitrogen combined complexes have larger population in experiment. The length of NO bond diminished in N combined complexes and it lengthened in O combined complexes. Still, the frequencies of NO in the former suffered blue shifts but did red shifts in the latter.

Key words: density functional theory, MP2, intermolecular complex, cytosine, nitrogen monoxide