Hartree-fock Study of Hydrogen-bonded Systems in the Absence of Basis-set Superposition Error the Nucleic-acid Base Pairs

The Roothaan equations have been recently modified for computing molecular interactions between weakly bonded systems at the SCF level in order to avoid the introduction of the basis-set superposition error (BSSE). Due to the importance that nucleic-acid base interactions play in DNA and RNA, which are 3-D structures, we present applications of this approach of Self Consistent Field for Molecular Interactions (SCF-MI) to the study of several hydrogen-bonded DNA bases. Nucleic-acid pairs are extensively investigated: structures and energies for both isolated and paired molecules are thoroughly studied. Cs-symmetry equilibrium geometries and binding energies are calculated in the framework of the SCF-MI formalism by using standard basis sets. SCF-MI/3-2 1G stabilisation energies for the studied base pairs lie in the range −22.5/−8.0 kcal/mol, in good agreement with previous, high-level theoretical values. The hydrogen bonding potential energy surface (PES) and propeller twist potentials are also calculated for some of the molecular complexes. The SCF-MI interaction density is used to interpret the nature of the interactions involved in the hydrogen bond formation: structure and stabilisation of the base pairs turn out to be determined mostly by electrostatic interactions. Preliminary calculations on stacked cytosine dimer are also reported. The SCF-MI/3-21G results show agreement with the counterpoise corrected SCF/6-31G** calculation