POTENTIAL ENERGY SURFACE OF THE STACK CYTOSINE DIMER: MP2 INTERACTION ENERGIES

M. Oltean1, R. Scholz2, D.R.T. Zahn2, V. Chiş1

1Babes-Bolyai University Faculty of Physics Kogalniceanu 1 RO-400084 Cluj-Napoca
2Institut für Physik, Technische Universität Chemnitz D-09107, Chemnitz, Germany 

In the last few years, various studies have focused on organic thin films due to their potentially interesting properties for electronic and optoelectronic applications such as light-emitting diodes, photovoltaic cells, field-effect transistors or waveguide-coupled detectors.
Among the molecules of interest, cytosine plays a special role because high quality films can be grown on different substrates with molecular beam deposition, and the resulting epilayers can be studied using a variety of techniques1,2. Experimental findings must be compared with quantum chemical investigations for a proper understanding of the mechanisms responsible for this interaction. However, calculations should be performed at an adequate theoretical level where various contributions to the stabilization energy are properly considered. From this perspective, it is well established that density functional methods, although suitable for planar base pairing, fail for base stacking3,4.

Cytosine dimer structures chosen in this study

For this reason, we investigated the interaction potential of the stack cytosine dimer at the second-order Møller – Plesset perturbation theory (MP2) level, in conjunction with split-valence basis sets augmented with polarization and diffuse functions and also with correlation consistent basis sets. 
In order to obtain the interaction potential for cytosine dimers we considered different stacked structures: parallel undisplaced (face-to-face), anti-parallel undisplaced (face-to-back) dimers and the intermediate structures. The vertical separation between the coplanar monomers was varied in each case between 3.0 and 4.5Å by a 0.1Å step, assuming rigid monomers whose geometries were either those from X-ray diffraction data or the gas-phase optimized structures at the same level of theory as that used for calculation of interaction energy. The relative stability of the different stacked structures is analyzed and discussed.
An important shift of the minimum of the interaction potential toward the larger separation is obtained by correcting the interaction energy for the Basis Set Superposition Error, simultaneous with a significant decrease in its depth. 
It is shown that the stacking interaction energy in cytosine dimer is very sensitive to the quality of the basis sets used in calculation. Thus, the correlation consistent basis sets give larger interaction energies when compared to the split valence basis sets. In addition, diffuse functions on heavy atoms systematically improve the stabilization energy of the dimer. 

1.	D.R.T. Zahn, G. Salvan, B.A. Paez, R. Scholz, J.Vac.Sci.Technol., 2004, 22, 1482
2.	M.A. Baldo, S.R. Forrest, Phys. Rev. B 2001, 64, 085201.
3.	P. Hobza, J.  Sponer, J. Am. Chem. Soc. 2002, 124, 11802
4.	P. Jurecka, J. Sponer, P. Hobza, J. Phys. Chem. B, 2004, 108, 5466-5471