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(June 30, 2021)

There is also another approximate method, one-center NOCIS (1C-NOCIS)
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Phys. Chem. Chem. Phys.

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, 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:

$n$
Non-negative 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 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