Large scale car-parrinello simulation of fully hydrated DNA

Density functional (BLYP/plane wave) calculations have been used to investigate the structure and the frontier orbitals of a fully hydrated crystalline DNA. Due to the size of the system (1194 atoms, 3,960 valence electrons requiring a total of 408,238 plane waves) this is one of the largest scale ab-initio simulations ever made. We find that the structure of water molecules around the DNA as well as their dipole moments are rather different from those of bulk water. The lowest conduction band is, surprisingly, found to be localized between the Na+ counter-ions and the PO4- groups. This gives rise to a gap of only 1.28 eV. We have evaluated the anisotropic optical conductivity. At low frequency the conductivity along the DNA axis is dominated by the pi --> Na+ transitions, while the in-plane optical activity is mostly intrabase ( pi --> pi* and n --> pi*). Our calculation demonstrates that hydration effects are fundamental for a proper understanding of DNA electronic properties.