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7.5 Non-Orthogonal Configuration Interaction (NOCI)

7.5.2 Job Control

(December 20, 2021)

Q-Chem 5.2 features a new package libnoci dedicated to running NOCI calculations, and accessed using USE_LIBNOCI = TRUE. The libnoci implementation introduces flexible job control, whereby the method used to generate multiple states for the NOCI basis can be defined by the user. Initially, a set of reference determinants must be chosen, either using a single SCF calculation or by reading sets or orbital coefficients from file as requested by the $rem variable NOCI_REFGEN. From these reference states, multiple non-orthogonal states are then created either using SCF metadynamics, or as excitations from the reference determinants, as requested by the $rem variable NOCI_DETGEN. When generating multiple using excitations within an active space, the active orbitals are controlled using the $active_orbitals input section. For example:

$active_orbitals
    1 4 11 14
$end

The β orbitals are offset by the total number of α molecular orbitals (e.g. the above example selects α orbitals 1 and 4, along with β orbitals 1 and 4, with a total of 10 α MOs including occupied and virtual orbitals). The resulting multiple determinants are then individually relaxed at the SCF level, unless this is explicitly skipped using SKIP_SCFMAN = TRUE. Finally, NOCI is run using the full set of multiple determinants identified.

The libnoci package also incorporates compatibility with the new SCF metadynamics implementation (see Section 4.9.3), as well as the holomorphic Hartree–Fock approach which allows multiple Hartree–Fock solutions to be continued across all geometries (see Section. 4.9.4).

USE_LIBNOCI

USE_LIBNOCI
       Turn on the use of libnoci for running NOCI calculations.
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       False Do not use libnoci (uses original Q-Chem implementation). True Use the libnoci implementation.
RECOMMENDATION:
       The $rem variables detailed below are only available in libnoci.

NOCI_REFGEN

NOCI_REFGEN
       Control how the initial reference determinants are created.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Generate initial reference determinant from a single SCF calculation. 1 Read (multiple) initial reference determinants from a previous calculation.
RECOMMENDATION:
       The specific reference determinants to be read from a previous calculation can be indicated using SCF_READMINIMA.

NOCI_DETGEN

NOCI_DETGEN
       Control how the multiple determinants for NOCI are created.
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Use only the initial reference determinants. 1 Generate CIS excitations from each reference determinant. 2 Generate all FCI excitations from each reference determinant. 3 Generate n multiple determinants using SCF metadynamics, where n is specified using SCF_SAVEMINIMA = n. 4 Generate all CAS excitations from each reference determinant, where the active orbitals are specified using the $active_orbitals input section.
RECOMMENDATION:
       By default, these multiple determinants are optimized at the SCF level before running NOCI. This behavior can be turned off using by specifying SKIP_SCFMAN = TRUE.

NOCI_NEIGVAL

NOCI_NEIGVAL
       The number of NOCI eigenvalues to be printed.
TYPE:
       INTEGER
DEFAULT:
       10
OPTIONS:
       n Positive integer
RECOMMENDATION:
       Increase this to print progressively higher NOCI energies.

NOCI_PRINT

NOCI_PRINT
       Specify the debug print level of NOCI.
TYPE:
       INTEGER
DEFAULT:
       1
OPTIONS:
       n Positive integer
RECOMMENDATION:
       Increase this for additional debug information.

Example 7.19  NOCI for H2 run in the libnoci implementation, with multiple determinants generated using SCF metadynamics.

$molecule
   0 1
   H    0.0000000   0.0000000   0.0000000
   H    0.0000000   0.0000000   4.0000000
$end

$rem
   EXCHANGE                   hf
   CORRELATION                noci
   UNRESTRICTED               true
   BASIS                      sto-3g
   SCF_CONVERGENCE            10
   MAX_SCF_CYCLES             1000
   MOM_START                  1
   USE_LIBNOCI                true
   SCF_SAVEMINIMA             4
   SCF_MINFIND_RANDOMMIXING   30000
   SCF_MINFIND_MIXMETHOD      1
   NOCI_REFGEN                0
   NOCI_DETGEN                3
   NOCI_NEIGVAL               4
$end

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