Homonuclear 3d transition-metal diatomics: A systematic density functional theory study

Abstract

The equilibrium bond lengths, harmonic vibrational frequencies, and dissociation energies of the ground state homonuclear 3d transition-metal diatomics (scandium through copper) were determined using six density functional or hybrid Hartree–Fock/density functional theory (HF/DFT) methods and unrestricted Hartree–Fock theory. Results are compared to other theoretical studies and to experimental values when available. The accuracy of the DFT results is found to be highly dependent upon the functional employed, with the pure DFT methods, BLYP and BP86, often performing significantly better than the hybrid HF/DFT methods. For the van der Waals complex Mn2, all six functionals predict the ground state to be high-spin, disagreeing with experiment; the true (antiferromagnetic) ground state was not found for any functional. Average errors for theoretical geometries and vibrational frequencies are for B3LYP, 0.053 Å (2.4%) and 122 cm−1 (31.1%); for B3P86, 0.051 Å (2.4%) and 122 cm−1 (31.3%); for BHLYP, 0.077 Å (4.1%) and 208 cm−1 (49.3%); for BLYP, 0.024 Å (1.3%) and 98 cm−1 (24.5%); for BP86, 0.020 Å (1.1%) and 104 cm−1 (25.6%); and for LSDA, 0.056 Å (3.0%) and 158 cm−1 (37.9%). No functional gives results directly comparable for all nine species. Dissociation energy results are severely overestimated in many instances and negative in others. Anecdotal reports of success for density functional theory for these systems may have been overblown.