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 [Seeger77, Bauernschmitt96](see also [Schlegel91a]). These include:

- Allowing an RHF determinant to become UHF.
- Allowing orbitals to become complex.
- Reducing the symmetry of the orbitals.

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. 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.

Note that analytic frequency calculations are only valid if the wavefunction has no internal instabilities. 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. Møller-Plesset calculations are only valid if the wavefunction has no internal instabilities within the constrained symmetry. 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 [Carsky91].

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.

#### RExt

Test for external real instability as well as internal instability (the default).

#### Int

Test for internal instability (a lower determinant with the same constraints) only.

#### RRHF

Constrain the wavefunction testing or reoptimization to be real, spin-restricted. Synonymous with Singlet.

#### RUHF

Constrain the wavefunction testing or reoptimization to be real, spin-unrestricted. Synonymous with Triplet.

#### CRHF

Allow testing for real to complex instabilities in spin-restricted wavefunctions.

#### CUHF

Allow testing for real to complex instabilities in spin-unrestricted wavefunctions.

#### Opt

If an instability is found, reoptimize the wavefunction with the appropriate reduction in constraints, repeating stability tests and reoptimizations until a stable wavefunction is found. RepOpt is a synonym for Opt. NoOpt prevents reoptimization and is the default. See also the QCOnly option below.

#### 1Opt

Redo the SCF once if an instability is detected.

#### Direct

Forces a direct calculation. This is the default.

#### MO

Forces a stability calculation using transformed two-electron integrals (i.e., in the MO basis).

#### AO

Forces a calculation using the AO integrals (written to disk), avoiding an integral transformation. The AO basis is seldom an optimal choice, except for small molecules on systems having very limited disk. It is the default when SCF=Conven is also specified.

#### InCore

Forces an in-core algorithm.

#### ICDiag

Forces in-core full diagonalization of the matrix formed in memory from transformed integrals. It implies the use of MO integrals.

#### QCOnly

Suppresses the use of the regular SCF procedure (i.e., non quadratic convergence: Link 502) during later SCF calculations of the Stable=Opt iterations. This is the default.

#### XQC

Try to use the regular SCF procedure (Link 502) before quadratic convergence SCF (Link 508) for later SCF calculations of the Stable=Opt iterations.

#### Restart

Restarts the calculation off the checkpoint file. Also implies SCF=Restart.