There is also another approximate method, one-center NOCIS (1C-NOCIS)713, 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:
Positive integer
RECOMMENDATION:
This variable determines the starting orbital for the calculation. As an example, for the oxygen K-edge in CO2, 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:
Positive integer
RECOMMENDATION:
This variable determines the number of references for the calculation. As an example, for the oxygen K-edge in CO2, the number of references would be would be 2 (two oxygen atoms), whereas for carbon it would be 1 (one carbon atom).
Example 7.22 NOCIS for the N K-edge of N2
$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
Example 7.23 STEX for the C K-edge of CN
$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
Example 7.24 1C-NOCIS for the O K-edge of O2
$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