BUILDING BRIDGES BETWEEN THEORY AND EXPERIMENT. Mark A. Spackman, Department of Chemistry, University of New England, Armidale NSW 2351, AUSTRALIA
The comparison between theory and experiment has been a recurring theme of charge density analyses over the past three decades, and today we no longer expect that "if an experimental and a theoretical deformation map agree the most probable interpretation is that they are both wrong".1 Nevertheless, we have reached the point where mere comparison no longer suffices. An important emerging issue in charge density analyses is to ascertain precisely what information can be obtained from the charge density experiment that could not have been derived, perhaps more reliably and economically, from theoretical calculations. Because of this it is timely to re-appraise the different roles of experiment and theory, to better understand their comparative advantages (and weaknesses!) and to explore ways in which both can contribute in a complementary fashion to yield more information together about a system than either could provide separately.
Various means of comparing theoretical and experimental charge density results will be reviewed in this lecture, with some examples taken from the recent literature, and some recent strategies using both together will be highlighted. It will be argued that we should perhaps be concentrating our efforts in directions where experiment and theory disagree the most, rather than focusing on their agreement (which, echoing the healthy scepticism above, is sometimes more apparent than real because of subtle uncertainties in either or both). In the realm of molecular crystals at least, this suggests that we begin to look seriously at the area of intermolecular interactions: their effect on the electron density and other properties, and obtaining meaningful estimates of their energies from the diffraction data. This would open a vast range of possible applications in highly active areas of modern chemistry such as supramolecular chemistry or zeolite catalysis, areas which are not yet accessible by accurate theoretical methods.
1. F.L. Hirshfeld , J. Mol. Struct. 130, 125 (1985)
(the comment was attributed to W.N. Lipscomb, 1971)