A well known limitation of the time dependent Hartree-Fock approximation is that electronic relaxation is not accounted for in the calculation of excitation energies. This relaxation can be understood as a correlation effect, since it is due to the simultaneous excitation of an electron and the relaxation of the remaining N-1 electrons (or rather orbitals) due to the “hole” left behind by the excited electron. On the other hand the relaxation is well described already at the Hartree-Fock level when the excited states are optimized separately, in the so-called ΔSCF approach.
The Static Exchange (STEX) approximation is a cheap way to incorporate relaxation effects into the calculation of core excitation spectra, where it is particularly important for excitation energies and transition probabilities. The method works by making a configuration interaction singles expansion of the excited states with a reference determinant optimized for core holes in a particular set of orbitals. The excited states are then not orthogonal to the Hartree-Fock ground state, and wavefunction overlaps have to be explicitly taken into account.
A STEX calculation typically proceeds in the following way:
Basic
Indicate (singly occupied) hole orbitals. Example:
.HOLES
! Expects format like (NFSYM=2)
2 ! nr of holes in fermion symmetry 1.
5 9 ! holes in fs 1
1 ! nr of holes in fermion symmetry 2.
1 ! holes in fs 2
Number of simultaneous Fock matrices to build. Default: 4
Deactivated ?
Set default cutoff in eV. Default: 100 eV
Screening threshold.