Bond dissociation restricted Hartree-Fock methods

The Restricted Hartree-Fock (RHF) method used to describe the dissociation of the chemical bond gives simply tragic results (cf. Chapter 8, p. 437), qualitatively wrong on the other hand, the Uruestricted Hartree-Fock (UHF) method gives a qualitatively correct description. 

All the calculations of F2 are carried out with a simple basis set of double-zeta polarization type, the standard 6-31G(d) basis set, and are performed at a fixed interatomic distance of 1.44 A, which is approximately the optimized distance for a full Cl calculation in this basis set. Only the corresponding orbitals are referred to as the active orbitals , while the orbitals representing the lone pairs, so-called spectator orbitals , remain doubly occupied in all calculations. A common point to the various VB methods we use, except the VBCI method, is that at the dissociation limit, the methods converge to two F fragments at the restricted-open-shell Hartree Fock (ROHF) level. 

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