RI-MP2 calculations with extended basis sets - a promising tool for study of H-bonded and stacked DNA base pairs

The interaction energies of 9-methyladenine . . .1-methylthymine H-bonded a nd 9-methyladenine . . . 9-methylguanine stacked pairs were evaluated at th e MP2 and resolution of the identity MP2 (RI-MP2) levels. The interaction e nergies are almost identical for both methods. The RI-MP2 method is about o ne order of magnitude faster than the exact MP2 method and, therefore, this method is well suited for the study of large models of biological importan ce. The basis set dependence of both base pairs was studied and reasonable relative energy characteristics were obtained only if at least the valence double-zeta -polarisation SVP basis set with modified (diffuse) polarisatio n functions on non-hydrogen and hydrogen atoms was used. However, reliable absolute stabilisation energies of both base pairs were only obtained if at least the aug-SVP basis set is applied. Stack interaction energies and thr ee- and four-body energy terms for selected base-pair steps of B-DNA were c alculated for the first time using the RI-MP2 method and the aug-SVP basis set.