MP2 and CCSD(T) study on hydrogen bonding, aromatic stacking and nonaromatic stacking

Three groups of molecular clusters were studied using the coupled cluster method with non-iterative triple excitations (CCSD(T)) and the second-order Mّller-Plesset perturbational method (MP2): H-bonded DNA base pairs ((cytosine)2, (isocytosine)2 and (uracil)2), aromatic stacked complexes ((pyrrol)2, (pyrimidine)2, (triazine)2, (aminotriazine)2, (4-aminopyrimidine)2 and (1-aminopyrimidine)2) and cyclic H-bonded and stacked (formamide)2 and (formamidine)2 dimers. The higher-order correlation energy contributions are repulsive in all aromatic stacked clusters, while for all other systems the CCSD(T) and MP2 methods provide nearly identical results. The interaction energies of stacked complexes converge with the size of basis set much faster than the interaction energies of H-bonded clusters. It follows from the present data that the stacking energies of nucleic acid base pairs, evaluated at the MP2 level with diffuse-polarized medium-sized basis sets should be close to the actual values. The stabilization of H-bonded base pairs evaluated at the same level of theory is expected to be underestimated.