Let us remind first that DIRAC uses the so-called averaged SCF scheme, which means that all the configurations belonging to the reference space receive the same weight.
In the case when one (or few) of the states from the reference space is (or are) more important then others, than the large open-shell space can lead to dis-balanced treatment and therefore to higher SCF energies than those obtained with a smaller reference space.
The “better” SCF configuration is that one giving the lowest total energy. Upon suitable averaged open-shell SCF occupation one can perform complete open-shell CI (COSCI) calculatons, one example is given below.
Calculate the spin-orbit components of the 2P ground state of the fluorine atom using a COSCI wavefunction. This will represent the “uncorrelated” result since the CI will merely give the energies of the individual determinants in this simple case.
The same calculation can also be done automatically by specifying .RESOLVE in the **WAVE FUNCTION section of the input.
The full calculation is given as input example of the cosci_energy test:
&GOSCIP NELEC=5, IPRNT=1 &END &POPANA THRESH=1.0D-4, DEGEN=1.0D-12, SELPOP=50.0 &END
NOTE: In the developer’s version the first line reads as
&GOSCIP NELACT=5, IPRNT=1 &END &POPANA THRESH=1.0D-4, DEGEN=1.0D-12, SELPOP=50.0 &END