DENSITY FUNCTIONAL STUDY OF AMMONIUM FLUORIDE: A COMPARISON BETWEEN THEORY AND EXPERIMENT Hans Overeem, Roelof Y. de Vries and Dick Feil, Chem. Phys. Lab. University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
In the molecular ion of NH4+ a large part of the electrons is involved in bonding. The electron density distribution is strongly influenced by the electric field of the F- ions.
The present study deals with quantum chemical density functional calculations on the crystal of NH4F. In molecular crystals, cluster calculations have been successful (Krijn et al, 1988). For ionic crystals, the cluster method meets serious problems: What is the potential? How to account for periodicity?
The present calculations were performed using Bloch functions to account for periodicity and a potential calculated with the Ewald method (program ADF-BAND (te Velde et al, 1990)). Dynamic structure factors were calculated and compared with multipole refined low-temperature data (van Beek et al, 1996). The difference amounted to R(F)=1.6%.
Realizing that multipole refinement not only reduces noise, but also introduces bias, the theoretical data were refined with the same POP-multipole program. This reduced the difference with experiment to R(F)=0.8%, the remainder for the greater part due to the NH4 group (R(F)=1.1%) and to a lesser part to the fluorine. Electron density maps corresponding with the various differences will be shown.
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C.G.van Beek, J.Overeem, J.R.Ruble and B.M.Craven, (1996) Canadian Journal of Chemistry, (accepted for publication).
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