Stability Analysis

Stability analysis of spin-restricted closed-shell ground states is enabled by

$scfinstab singlet

for singlet instabilities,

$scfinstab triplet

for triplet instabilities (most common), and

$scfinstab non-real

for non-real instabilities.

After that, it is necessary to specify the IRREPs of the electronic Hessian eigenvectors (``orbital rotations'') to be considered. Without additional knowledge of the system one usually needs to calculate the lowest eigenvalue within every IRREP:

$soes all 1

Positivity of the lowest eigenvalues in all IRREPs is sufficient for stability of the ground state solution. If one is interested in, say, the lowest eigenvalues in IRREPs eg and t2g only, one may specify:

$soes
eg   1
t2g  1

Triplet instabilities in the totally symmetric IRREP indicate open shell diradical states (singlet or triplet). In this case, start MOs for spin-symmetry broken UHF or UKS ground state calculation can be generated by specifying

$start vector generation

escf will provide the start MOs ( → $uhfmo_alpha, $uhfmo_beta) as well as occupation numbers ( → $alpha shells, $beta shells) for a spin-unrestricted calculation with equal numbers of α and β electrons (pseudo-singlet occupation).