Description

The SCF calculation (definition of the wavefunction) failed to converge. There are a number of possible reasons why this might occur, but they are usually related in one way or another to charge (electron) oscillations. In a system where a biradical can form, such as ethane decomposing into two CH₃ units or a hydrogen shift, the normal RHF procedure can fail to achieve self-consistence. If the system has significant biradicaloid character, BIRADICAL or UHF/TRIPLET will often prove successful in circumventing the error condition.

This problem can be demonstrated with H-Cl by steadily increasing the interatomic distance. At bonding distance the covalent bond will be strong, and a self-consistent field is readily obtained. As the bond is stretched, it becomes increasingly ionic. Electrons are polarized toward the chlorine and the hydrogen becomes electropositive. As the energy increases electrons are attracted by the positive charge on the hydrogen. At some point, an electron will suddenly “jump” from the chlorine to the hydrogen causing a re-organization of the density matrix. There is a good chance the electron will oscillate between the hydrogen and the chlorine, and SCF will not be achieved This can be prevented by using BIRADICAL, and in this instance exactly one electron will end up on hydrogen. A similar result can be obtained by specifying TRIPLET in a UHF calculation.

If this message does appear, suspect first that the input calculation requested might be faulty. If after examination the input appears correct, consider slackening SCFCRT. This will allow heats of formation to be calculated with reasonable precision, but the gradients are likely to be imprecise.