General Performance Overview of Ab Initio HF Theory

Koopmans theorem can be formally applied to electron affinities (EAs) as well, i.e., the EA can be taken to be the negative of the orbital energy of the lowest unoccupied (virtual) orbital. Here, however, relaxation effects and correlation effects both favor the radical anion, so rather than canceling, the errors are additive, and Koopmans theorem estimates will almost always underestimate the EA. It is thus generally a better idea to compute EAs from a ASCF approach whenever possible. 

One method proposed to correct for this phenomenon is the so-called counterpoise (CP) correction. Although some variations exist, one popular approach defines the CP corrected 

Geometries of TS structures are not readily available from experiment, but a fairly substantial body of theoretical work permits comparisons to be made with very high-level 

As for the energies of non-bonded complexes, the failure of HF theory to account for dispersion tends to make such complexes too loose in structure, i.e., intermolecular distances are unrealistically large. Hydrogen bonded structures, on the other hand, are often quite good because errors in overestimating electrostatic interactions cancel the failure to account for dispersion. The HF structures show the expected preference for linear bond angles at hydrogen, when such are possible, and further exhibit reasonable distances between donor and acceptor heavy atoms in most instances. 

---