Gaussian 03 Online Manual
This calculation type method requests that the stability of the Hartree-Fock or DFT wavefunction be tested. Gaussian has the ability to test the stability of a single-determinant wavefunction with respect to relaxing various constraints [106,107] (see also ). These include:
The default is to test for all instabilities but not to re-optimize the wavefunction. If Stable=Opt is specified, by default the wavefunction is allowed to be unrestricted if necessary. In examining the results prior to a frequency calculation, it suffices to see if any singlet instabilities exist for restricted wavefunctions or if any instabilities (singlet or triplet) exist for unrestricted wavefunctions. In examining the results prior to a Møller-Plesset calculation, an internal instability only affects the validity of the results if the pairs of orbitals mixed are of the same spatial symmetry. The validity of restricted Møller-Plesset energies based on wavefunctions which are unstable with respect to becoming UHF is also questionable .
The Stable keyword causes the program to compute a wavefunction as usual and then to determine if the resulting determinant is a local minimum with the specified degrees of freedom taken into consideration. Note that analytic frequency calculations are only valid if the wavefunction has no internal instabilities, and Møller-Plesset calculations are only valid if the wavefunction has no internal instabilities within the constrained symmetry. By default, only real instabilities (i.e., not complex) are sought. The code which checks for a complex stability (Link 902) is older and less reliable and should not be used unless complex orbitals are of interest.
for internal instability (a lower determinant with the same constraints) only.
WAVEFUNCTION REOPTIMIZATION OPTIONS
HF and DFT methods.