A CCSD(T) study of the relative stabilities of cytosine tautomers

Coupled-cluster (CC) calculations at the single, double (SD), and perturbative triple (T) excitation level have been carried out to establish the relative stabilities of the tautomers of cytosine. The basis set used was of triple-zeta quality, making this study the highest quality approach to date. The sensitivity of the results is discussed in terms of geometry, basis set, and method and in light of previous studies. In particular, the order of stability of the tautomers at the CCSD level found in a recent study by Fogarasi is found not to be the case at the CCSD(T) level. Comparison is also made with B3LYP density-functional theory results.