BASE STACKING IN CYTOSINE DIMER - A COMPARISON OF CORRELATED AB-INITIO CALCULATIONS WITH 3 EMPIRICAL POTENTIAL MODELS AND DENSITY-FUNCTIONAL THEORY CALCULATIONS

Ab initio MP2/6-31G interaction energies were calculated for more tha n 80 geometries of stacked cytosine dimer. Diffuse polarization functi ons were used to properly cover the dispersion energy. The results of ab initio calculations were compared with those obtained from three el ectrostatic empirical potential models, constructed as the sum of a Le nnard-Jones potential (covering dispersion and repulsion contributions ) and the electrostatic term. Point charges and point multipoles of th e electrostatic term were also obtained at the MP2/6-31G level of the ory. The point charge MEP model (atomic charges derived from molecular electrostatic potential) satisfactorily reproduced the ab initio data . Addition of pi-charges localized below and above the cytosine plane did not affect the calculated energies. The model employing the distri buted multipole analysis gave worse agreement with the ab initio data than the MEP approach. The MP2 MEP charges were also derived using lar ger sets of atomic orbitals: cc-pVDZ, 6-311 + G(2d,p), and aug-cc-pVDZ . Differences between interaction energies calculated using these thre e sets of point charges and the MP2/6-31G charges were smaller than 0 .8 kcal/mol. The correlated ab initio calculations were also compared with the density functional theory (DFT) method. DFT calculations well reproduced the electrostatic part of interaction energy. They also co vered some nonelectrostatic short-range effects which were not reprodu ced by the empirical potentials. The DFT method does not include the d ispersion energy. This energy, approximated by an empirical term, was therefore added to the DFT interaction energy. The resulting interacti on energy exhibited an artifact secondary minimum for a 3.9-4.0 A vert ical separation of bases. This defect is inherent in the DFT functiona ls, because it is not observed for the Hartree-Fock + dispersion inter action energy. (C) 1996 by John Wiley Sons, Inc.