Potential Energy Surface of the Cytosine Dimer: MP2 Complete Basis Set Limit Interaction Energies, CCSD(T) Correction Term, and Comparison with the AMBER Force Field

The complete basis set (CBS) limit of the MP2 interaction energy and the CCSD(T) correction term determined
as the difference between the CCSD(T) and MP2 interaction energies were evaluated for 17 stacked and 4
H-bonded structures of the cytosine dimer. Extrapolation to the MP2 CBS limit was done using the aug-cc-pVDZ and aug-cc-pVTZ results, and the CCSD(T) correction term was determined with the 6-31G*(0.25)
basis set. Extrapolation to the CBS limit is essential in all parts of the potential energy surface and affects
mainly the absolute MP2 stabilization energy. The effect on the relative stability is smaller but not negligible.
The CCSD(T) correction term is for stacked structures with large overlaps of the monomers substantially
repulsive but not uniform. Thus, when comparing the final estimate (abbreviated as CBS(T)) with previously
used medium-level MP2/6-31G*(0.25) data, the CCSD(T) correction partially compensates for the enhanced
absolute MP2 CBS stabilization but further increases the relative differences between different structures.
The difference between the MP2/6-31G*(0.25) and the new CBS(T) reference values is in the range of +0.3
to −2.1 kcal/mol. The difference between the parallel and antiparallel structures is enhanced by 2 kcal/mol.
The CCSD(T) correction is negligible in the H-bonded structure and in stacked structures with a minimal
overlap of bases. The new reference CBS(T) data are compared with the AMBER force field testing both HF
and MP2 electrostatic potential fitted atom-centered charges. The overall agreement of the force field with
the CBS(T) data is very satisfactory. We nevertheless identify differences that are attributed to polarization
and short-term effects not included in the force field. Interestingly, whereas MP2/6-31G*(0.25) calculations
are better reproduced with the MP2 variant of the force field the new CBS(T) reference data are in better
agreement with the use of HF charges in the force field, indicating the enhanced role of polarization.