There is also another approximate method, one-center NOCIS (1C-NOCIS)
Phys. Chem. Chem. Phys.
(2020), 22, pp. 8182. , which is an intermediate between NOCIS and STEX. The open-shell determinants are separately optimized as in NOCIS, but the coupling between non-orthogonal determinants with core holes on different centers is ignored, and NOCI is used to compute the remaining matrix elements between non-orthogonal determinants. 1C-NOCIS constructs the orthogonal Slater-Condon components of the matrices, and then performs NOCI to obtain the relevant non-orthogonal components. The diagonal blocks are then projected against the ground state. For singlet NOCIS, 1C-NOCIS is the same as STEX, since there are no open-shell ground-state orbitals.
There are two main advantages of 1C-NOCIS. First, it is substantially cheaper to evaluate than NOCIS and so enables the treatment of larger molecules. Second, and in contrast to STEX, it allows the open-shell states to relax separately, which may have a substantial impact on accuracy.
NOCIS, 1C-NOCIS, and STEX run as subroutines in GEN_SCFMAN. Thus, for all calculations, you must set GEN_SCFMAN = TRUE. It is also highly recommended that you run an open-shell ground state calculation prior to running your NOCIS/STEX/1C-NOCIS calculations, which are all restricted.
$molecule 0 1 N 0.000000 0.000000 0.564990 N 0.000000 0.000000 -0.564990 $end $rem METHOD hf BASIS sto-3g UNRESTRICTED false GEN_SCFMAN true NOCIS 1 ORB_OFFSET 0 NUM_REF 2 THRESH 14 SYMMETRY false SYM_IGNORE true MAX_SCF_CYCLES 500 $end
$molecule 0 2 C 0.0000000 0.0000000 -0.6258140 N 0.0000000 0.0000000 0.5364120 $end $rem METHOD hf BASIS sto-3g SCF_GUESS core SCF_ALGORITHM diis_gdm SYMMETRY false SYM_IGNORE true MAX_SCF_CYCLES 5000 THRESH 14 SCF_CONVERGENCE 10 $end @@@ $molecule read $end $rem METHOD hf BASIS sto-3g SCF_GUESS read UNRESTRICTED false SCF_ALGORITHM diis_gdm GEN_SCFMAN true STEX 1 ORB_OFFSET 1 NUM_REF 1 SYMMETRY false SYM_IGNORE true MAX_SCF_CYCLES 5000 THRESH 14 SCF_CONVERGENCE 10 $end
$molecule 0 3 O 0.0000000 0.0000000 0.6021380 O 0.0000000 0.0000000 -0.6021380 $end $rem METHOD hf BASIS sto-3g GEN_SCFMAN true THRESH 14 SYMMETRY false SYM_IGNORE true MAX_SCF_CYCLES 500 $end @@@ $molecule read $end $rem UNRESTRICTED false SCF_GUESS read METHOD hf BASIS sto-3g GEN_SCFMAN true NOCIS 1 ONE_CENTER 1 ORB_OFFSET 0 NUM_REF 2 THRESH 14 SYMMETRY false SYM_IGNORE true MAX_SCF_CYCLES 500 $end