Note on the generation of a new basis set for band structure calculations of infinite stacks

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The band structures of a formamide stack (in the DNA B stacking arrangement), as well of cytosine (C) and uracil (U) stacks were calculated at the ab initio Hartree–Fock (HF) level using first a double basis set. The resulting valence and conduction bands were corrected for correlation on the basis of the inverse Dyson equation in the diagonal approximation with a MP2 self-energy taking into account the relaxation effects.It has been found that if one supplements the double ζ basis with one belonging to a “phantom” molecule (a molecule without nuclear charges and electrons at the midway of the stacking distance, 3.36Å/2=1.68Å and at half the rotation angle, 18°) one obtains substantially larger correlation effects and the fundamental gap gets close to the values which can be estimated on the basis of the optical spectra. Most probably this finding will turn out to be important for other stacked systems.