Lattice dynamics of bromine and chlorine using the simple bond charge model

Method of increments for the halogen molecular crystals: Cl, Br, and I

Method of increments (MI) calculations reveal the n-body correlation contributions to binding in solid chlorine, bromine, and iodine. Secondary binding contributions as well as d-correlation energies are estimated and compared between each solid halogen. We illustrate that binding is entirely determined by two-body correlation effects, which account for >80% of the total correlation energy. One-body, three-body, and exchange contributions are repulsive. Using density-fitting (DF) local coupled-cluster singles, doubles, and perturbative triples for incremental calculations, we obtain excellent agreement with the experimental cohesive energies. MI results from DF local second-order Møller-Plesset perturbation (LMP2) yield considerably over-bound cohesive energies. Comparative calculations with density functional theory and periodic LMP2 method are also shown to be less accurate for the solid halogens.

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

The principle of zero net crystal force has been used to examine the inter-molecular forces operative in solid chlorine, Cl 2 . 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 Pa 3 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.