The first complete theoretical analysis of the gas-phase formation of
a nucleic acid base pair (uracil dimer) has been performed. The study
is based on a combination of AMBER 4.1 empirical potential, correlated
ab initio quantum chemical methods, computer simulations, and statistical
thermodynamics methods.In total, 11 low-energyminima structures were
located on the potential energy surface of uracil dimer: seven of them
are H-bonded, one is T-shaped, and three correspond to various stacked
arrangements. The most stable structure is a H-bonded dimer with two
N1-H...O2 H-bonds, designated as HB4; it has an energy minimum of 
-15.9 kcal/mol at the MP2/6-31G*(0.25)//HF/6-31G** level of theory.
T-shape structure and stacked structures are less stable than H-bonded
ones. Thermodynamic characteristics were obtained using the rigid rotor-
harmonic oscillator-ideal gas (RR-HO-IG) approximation adopting the AMBER 4.1
and ab initio characteristics.Furthermore, the population of various structures
was determined by computer simulations in the NVT canonical and NVE
microcanonical ensembles. Results obtained from the RR-HO-IG approximation
and the NVT ensemble are very similar and differ from the results of the
NVE ensemble. The present analysis demonstrates that different gas-phase
experimental techniques can be used for investigating different regions
of the conformational space for nucleic acid base pairs. Teh fact that
entropy is always significant and differs for H-bonded and stacked
structures is of importance.