PROPERTIES OF THE PROMOLECULE. B. Etschmann, E. N. Maslen. Crystallography Centre, The University of Western Australia, Nedlands, 6907, Australia
The power of the promolecule model, which consists of a (Hartree) product of the free atom wave functions, can be partly attributed to the number of chemical and physical properties that are unique functionals of the one-electron density1.
Evaluating atomic charges on overlapping neutral atoms in a promolecule may appear anomalous, but the close relationship between bonding and potential energies makes it physically reasonable to subdivide electron density in proportion to each atom's contribution to the electrostatic potential.
Atomic radii are determined in two stages. An invariant component of the radius associated with the atomic cores is first equated to the value at which the integral of the density equals the number of core electrons. The second most significant contribution is from the valence electrons, which must be treated as penetrable. The main requirement when evaluating atomic radii from atomic electron densities is to evaluate the penetrability of the valence subshells.
The promolecule also gives a good first order prediction of bonding energies for atoms extending across the whole periodic table2. Energies predicted for diatomic molecules containing a monovalent anion an cation at the equilibrium spacing approximate bonding energies more accurately than integral point charges at those locations.
Values for these properties predicted by the promolecule model will be compared with experimentally measured equivalents for the garnet Yb3Al5O12.
(1) Hohenburg, P.& Kohn, W. Phys. Rev. 1964, B136, 864-871
(2) Spackman, M. A.& Maslen, E. N. J. Phys. Chem. 1986, 90, 2020-2027