Existence of additional solutions to the equations of the Hartree
–Fock method occurring usually in the case when potential energy surfaces of different
electronic states are drawn close together. Within the spin-restricted Hartree
–Fock method (RHF
), singlet and triplet instabilities are distinguished. The former involves the existence
of another solution with lower energy and the electron distribution of lower symmetry.
It may be regarded as an indication that the initially assumed molecular shape
needs correction. The triplet instability involves rejection of the condition of
double occupancy of molecular orbitals and thus transition to the spin-unrestricted
–Fock method (UHF
). The triplet instability is a necessary, but insufficient, condition for the conclusion
as to the biradical
character of the ground state
of a given system.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by
A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997).
XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic,
J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. https://doi.org/10.1351/goldbook