Preferred Label : instability of Hartree–Fock solution;
IUPAC acronym : RHF; UHF;
IUPAC definition : 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 Hartree–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.;
Origin ID : IT07056;
See also
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 Hartree–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.
