7.7 Non-Orthogonal Configuration Interaction with Single Substitutions (NOCIS) and Static Exchange (STEX)

7.7.3 One-Center NOCIS (1C-NOCIS)

(June 30, 2021)

There is also another approximate method, one-center NOCIS (1C-NOCIS) 818 Oosterbaan K. J. et al.
Phys. Chem. Chem. Phys.
(2020),
Link
, 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.

7.7.3.1 Job Control

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 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:
       n 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 7.24  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
   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

View output

Example 7.25  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
   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

View output

Example 7.26  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
   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

View output