QUANTUM CHEMICAL AND EXPERIMENTAL STUDY OF UREA. Dirk Feil and Roelof de Vries, Chem. Phys. Lab, University of Twente P.B 217, 7500 AE Enschede, Netherlands; Vladimir Tsirelson, D. Mendeleev University of Chemical Technology, Moscow, Russia

Introduction

In urea crystals one finds a considerable interaction density due to the network of strong hydrogen bonds distorting the molecular electron density distribution. The infinite size of the network requires, strictly speaking, band structure calculations to determine this effect.

Crystal Hartree-Fock-calculations (HF) have been carried out by Dovesi et al while DFT-calculations on clusters and on the crystal were carried out with ADF and the Amsterdam BAND program, respectively, as part of the present study. The outcome of the calculations are compared with the experimental results of Swaminathan et al and of Zavodnik et al.

Results

The difference between the HF and the DFT exceeds experimental uncertainty.

The description of the atoms is an important part of the discrepancy between HF and experiment.

The interaction densities, resulting from HF and DFT calculations, do not differ significantly.

Band structure calculations are not essential for the determination of the interaction density; with well-chosen parts of many-molecule clusters one can construct molecules that hardly differ from molecules in the crystal. Cluster calculations allow the use of larger basis sets.

References

R. Dovesi et al. J.Chem. Phys. 92, 7402 (1990)

G. te Velde, E.J.Baerends, Phys. Rev B44, 7888 (1991)

S. Swaminathan et al. Acta Cryst. B40, 398 (1984)

Zavodnik, Poster contribution to the present conference