Accurate structures and binding energies for stacked uracil dimers

The face-to-face and face-to-back stacked uracil dimers have been investigated by second-order Moller-Plesset (MP2) perturbation theory and by the coupled-cluster singles and doubles method augmented with a perturbative contribution from connected triple substitutions [CCSD(T)]. Full MP2 geometry optimizations were per-formed with a TZ2P(f,d)++ basis and with the 6-31G* basis for which harmonic vibrational frequencies were computed as well. Complete basis set MP2 binding energies were obtained from basis set extrapolations using the correlation-consistent basis sets cc-pVXZ (X = D-5) and aug-cc-pVXZ (X = D-Q). Higher-order correlation effects were gauged by computing the MP2 --> CCSD(T) shift in the counterpoise-corrected binding energy using a modified 6-31G* basis set. By adding this correction to the infinite basis set limit MP2 binding energies, final estimates of 9.7 and 8.8 kcal mol(-1) are obtained for the binding energies of the face-to-face and face-to-back structures, respectively.