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Anisotropy of van der Waals radii of atoms in molecules: alkali-metal and halogen atoms

The van der Waals radii of the alkali-metal and halogen atoms in 28 diatomic molecules are estimated theoretically, where the radius is defined as the distance from the nucleus to the surface having a calculated electron density of 0.005a. u. (0.0337 e A-3). The anisotropy of their radii is studied. The electron-correlation effect on the electrondensity distribution in Na + and F increases slightly (by 0.01-0.02/~) the radii calculated at the HartreeFock level of theory. A large 'polar flattening' is observed for the CI and Br atoms bonded to H or halogen atoms. This can arise from the fact that the Pz electrons of the C1 and Br atoms flow into the bonding region in forming the chemical bond. On the other hand, a small 'polar extension' is observed for the K and Na atoms bonded to halogen atoms. This phenomenon is described with a simple model calculation (Na + . . . . l e), where the F atom in an NaF molecule is replaced with a negative point charge. The larger repulsion energy of an (HF)2 dimer in a sideways-on contact than in a head-on contact reflects the anisotropy of the electron-density distribution around the F atom in the molecule.