Influence of intermolecular interactions on multipole-refined electron densities

This work examines the effect of intermolecular interactions on molecular properties derived from simulated X-ray diffraction data.

Model X-ray data are computed from a superposition of ab initio

molecular electron densities in the crystal, as well as from periodic

crystal Hartree-Fock electron densities, for the hydrogen-bonded

systems ice VIII, formamide and urea, as well as the weakly bound

acetylene. The effects of intermolecular interactions on the electron

density are illustrated at both infinite and finite data resolution,

and it is concluded that multipole models are capable of quantitative

retrieval of the interaction density, despite the known shortcomings of

the radial functions in the model. Multipole refinement reveals

considerable enhancement of the molecular dipole moment for

hydrogen-bonded crystals, and negligible change in molecular second

moments. Electric field gradients at H nuclei are significantly reduced

in magnitude upon hydrogen bonding, and this change is also faithfully

represented by the rigid pseudoatom model.