Effects of the magnetic part of the Breit term on bonding: Model calculations with small diatomic molecules

Model calculations for small molecules Li-2, F-2, LiF and BF have been perf ormed at the Dirac-Fock level of theory using Dirac-Coulomb and Dirac-Coulo mb-Magnetic Hamiltonians with various basis sets. In order to understand wh at may happen when the relativity becomes significant, the value of c, spee d of light, is varied from the true value of 137.036 a.u. to 10(5) (nonrela tivistic case) and also to 50 and 20 a.u. (exaggerated relativistic cases). Qualitative trends are discussed with special emphasis on the effect of th e magnetic part of the Breit interaction term. The known relativistic effec ts on bonding such as the bond length contraction or expansion are demonstr ated in dris model study. Total energy, pi -orbital splitting, bond length, bond dissociation energy and dipole moment are calculated, and shown to be modified in a uniform direction by the effect of the magnetic term. Inclus ion of the magnetic term raises the total energy, increases the bond length , reduces the pi -orbital splitting, increases the bond dissociation energy , and mitigates the changes in dipole moment caused by the Dirac term.