Quantum field theoretical methods in chemically bonded systems III

Summary

A new perturbative method is applied to single bonds. The starting model is the second-quantized self-consistent Heitler-London model. The unperturbed function is a four-determinant Bardeen-Cooper-Schrieffer function. Perturbative corrections are computed with renormalized Feynman diagrams. Convergence is satisfactory by third order. Calculated (experimental) dissociation energies in eV are 4.61 (4.75) for H₂, 2.37 (2.52) for LiH, 6.22 (6.13) for FH, and 1.88 (1.66) for F₂. Calculated (experimental) equilibrium bond distances in Å are 0.739 (0.741) for H₂, 1.598 (1.596) for LiH, 0.903 (0.917) for FH, and 1.395 (1.412) for F₂. Calculated (experimental) vibrational frequencies in cm⁻¹ are 4578 (4401) for H₂, 1396 (1406) for LiH, 4447 (4138) for FH, and 927 (916) for F₂. Other spectroscopic constants agree with experiment to within 11% except for anharmonicities which differ from experiment by up to 20%.