The dispersion interaction between a pair of parallel DNA double-helix structures is investigated by means of the van der Waals density functional (vdW-DF) method. Each double-helix structure consists of an infinite repetition of one B-DNA coil with 10 base pairs. This parameter-free density functional theory (DFT) study illustrates the initial step in a proposed vdW-DF computational strategy for large biomolecular problems. The strategy is to first perform a survey of interaction geometries, based on the evaluation of the van der Waals (vdW) attraction, and then limit the evaluation of the remaining DFT parts (specifically the expensive study of the kinetic-energy repulsion) to the thus identified interesting geometries. Possibilities for accelerating this second step is detailed in a separate study. For the B-DNA dimer, the variation in van der Waals attraction is explored at relatively short distances (although beyond the region of density overlap) for a 360 degrees rotation. This study highlights the role of the structural motifs, like the grooves, in enhancing or reducing the vdW interaction strength. We find that to a first approximation, it is possible to compare the DNA double strand at large wall-to-wall separations to the cylindrical shape of a carbon nanotube (which is almost isotropic under rotation). We compare our first-principles results with the atom-based dispersive interaction predicted by DFT-D2 [J. Comp. Chem. 27, 1787 (2006)] and find agreement in the asymptotic region. However, we also find that the differences in the enhancement that occur at shorter distances reveal characteristic features that result from the fact that the vdW-DF method is an electron-based (as opposed to atom-based) description.
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