There is also another approximate method, one-center NOCIS (1C-NOCIS)^{Oosterbaan:2020}, 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.

NOCIS

Run a NOCIS calculation

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

False
Do not run a NOCIS calculation.
True
Run a NOCIS calculation.

RECOMMENDATION:

This variable must be set to true to run a NOCIS or a 1C-NOCIS calculation.

STEX

Run a STEX calculation

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

False
Do not run a STEX calculation.
True
Run a STEX calculation.

RECOMMENDATION:

This variable must be set to true to run a STEX calculation. NOCIS cannot be set to true.

ONE_CENTER

Run a 1C-NOCIS calculation

TYPE:

LOGICAL

DEFAULT:

FALSE

OPTIONS:

False
Run a NOCIS calculation.
True
Run a 1C-NOCIS calculation.

RECOMMENDATION:

This variable must be set to true to run a 1C-NOCIS calculation, and NOCIS must be set to true as well.

ORB_OFFSET

Determine the starting orbital for a NOCIS/STEX/1C-NOCIS calculation

TYPE:

Integer

DEFAULT:

None

OPTIONS:

$n$
Positive integer

RECOMMENDATION:

This variable determines the starting orbital for the calculation. As an example, for the oxygen K-edge in CO_{2}, the starting orbital would be 0, whereas for carbon it would be 2.

NUM_REF

Set the number of atoms (references) to be included in the excitation calculation

TYPE:

Integer

DEFAULT:

None

OPTIONS:

$n$
Positive integer

RECOMMENDATION:

This variable determines the number of references for the calculation. As an example, for the oxygen K-edge in CO_{2}, the number of references would be would be 2 (two oxygen atoms), whereas for carbon it would be 1 (one carbon atom).

$molecule 0 1 N 0.000000 0.000000 0.564990 N 0.000000 0.000000 -0.564990 $end $rem unrestricted false jobtype sp method hf basis sto-3g 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 jobtype sp method hf scf_guess core scf_algorithm diis_gdm basis sto-3g symmetry false sym_ignore true max_scf_cycles 5000 thresh 14 scf_convergence 10 $end @@@ $molecule read $end $rem jobtype sp method hf basis sto-3g unrestricted false scf_guess read 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 jobtype sp 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 jobtype sp 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