Finding symmetry breaking Hartree-Fock solutions: The case of triplet instability

by Toth, Z.; Pulay, P.

Determining the lowest unrestricted Hartree-Fock (UHF) solution is often difficult in even-electron systems. We have developed a deterministic method for locating approximately the UHF minimum using the restricted Hartree-Fock triplet instability matrix. The current method is truncated to fourth order. The minimum energy solution for this model can be determined by solving a small linear system of equations. This solution gives a suitable starting point to determine the exact UHF solution. This should be useful for the black-box determination of active spaces spanned by the fractionally occupied charge natural orbitals of the ground-state UHF wavefunction. The results can be generalized to higher (6th and 8th) degree expansions (odd expansion orders vanish by symmetry), and to other types of instability, including complex instability. The results are illustrated by calculations on ozone, benzene, nitrobenzene, butadiene, hexatriene, octatetraene, dichromium, and nickel porphine. Further examples are given in the supplementary material. Published by AIP Publishing.

Journal
Journal of Chemical Physics
Volume
145
Issue
16
Year
2016
URL
https://dx.doi.org/10.1063/1.4964903
ISBN/ISSN
1089-7690; 0021-9606
DOI
10.1063/1.4964903