A comparison of procrystal and ab initio model representations of the electron-density distributions of minerals |
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Authors: | R T Downs G V Gibbs M B Boisen Jr K M Rosso |
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Institution: | (1) Departments of Geosciences, Materials Science and Engineering and Mathematics, Virginia Tech, Blacksburg, VA 24061, USA;(2) Department of Chemical Engineering, Virginia Tech, Blacksburg, VA 24061, USA;(3) W.R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, KB-96, Richland, WA 99352, USA;(4) Mineralogisch-Petrologisches Institut, Universität Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany;(5) GKSS, Max-Planck-Strasse, 21502 Geesthacht, Germany;(6) Institut für Materialchemie, Technische Universität Wien, Getreidemarkt 9/156-TC, 1060 Wien, Austria |
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Abstract: | The procrystal calculation of the electron density is a very rapid procedure that offers a quick way to analyze various bonding
properties of a crystal. This study explores the extent to which the positions, number, and properties of bond-critical points
determined from the procrystal representations of the electron density for minerals are similar to those of first-principles
ab initio model distributions. The purpose of the study is to determine the limits imposed upon interpretation of the procrystal
electron density. Procrystal calculations of the electron density for more than 300 MO bonds in crystals were compared with those previously calculated using CRYSTAL98 and TOPOND software. For every bond-critical
point found in the ab initio calculations, an equivalent one was also found in the procrystal model, with similar magnitudes
of electron density, and at similar positions along the bonds. The curvatures of the electron densities obtained from the
ab initio and the procrystal distributions are highly correlated. It is concluded that the procrystal distributions are capable
of providing good estimates of the bonded radii of the atoms and the properties of the electron-density distributions at the
bond-critical points. Because the procrystal model is so fast to compute, it is especially useful in addressing the question
as to whether a pair of atoms is bonded or not. If the Bader criteria for bonding are accepted, then the successful generation
of the bond-critical points by the procrystal model demonstrates that bonding is an atomic feature. The main difference between
the critical-point properties of the procrystal and the ab initio model is that the curvature in the electron density perpendicular
to the bond path of the ab initio model is sharper than for the procrystal model. This is interpreted as indicating that the
electrons that migrate into a bond originate from its sides, and not from the regions closer to the nuclei. This observation
also suggests that ab initio optimization routines could see an improvement in speed if the parameters relating to the angular
components of atomic wave functions were to vary before the radial components.
Received: 6 August 2001 / Accepted: 21 November 2001 |
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Keywords: | Electron density Procrystal Bond-critical points Pyroxene Feldspar Topology |
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