Anisotropic atom–atom forces and the space group of solid chlorine

Abstract

The principle of zero net crystal force has been used to examine the inter-molecular forces operative in solid chlorine, Cl₂. A spherical atom–atom 6:12 potential does not predict the observed space group, orthorhombic Cmca nor does the inclusion of molecular point quadrupole interactions. If the difference electron density is calculated from an s. c. f. wavefunction with large basis and used to calculate the electrostatic interaction by numerical integration Cmca is significantly stabilized but the cubic Pa3 is more stable. When the spherical atom–atom potential is replaced by one which is flattened at the atomic pole and equator, not only does Cmca become the correctly predicted space group, but it is free from internal crystal forces and the cell parameters change little on relaxation.