Advances in molecular modeling: oriented orbitals, and a technique for improving data quality

Abstract

<p>Standard procedures for modeling electron density in crystalline solids relies on the superimposition of spherical (isolated) atoms upon the nuclear sites. This approach works remarkably well and provides a good first approximation to the electron density. Beyond this there are a myriad of approximations ranging from superimpositions of oriented ground-state atoms upon a molecule's atomic sites to usage of molecular wavefunctions to model the experimentally determined electron density;The method described here focuses on promolecules which are formed from the superimposition of atoms upon molecules which have their valence orbitals oriented. Improved fits to the observed density is demonstrated and the additional information these enhanced models provide is discussed;X-ray intensity data collected from real crystals suffer from absorption to varying degrees. Failure to correct for absorption results in systematic errors in the resultant molecular model's parameters and an overall poorer fit to measured data. A mathematical technique which calculates correction terms using only data collected under normal circumstances is described. The results for several compounds are shown and general features discussed. ftn[superscript] aDOE Report IS-T-1341. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.</p>