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12.17 Ab Initio Frenkel Davydov Exciton Model (AIFDEM)

12.17.2 Job Control Variables

(December 20, 2021)

A basic AIFDEM calculation is requested by setting AIFDEM = TRUE in the $rem section. Additional job control variables dictate which types of states are included in the exciton-site basis as well as the NTO threshold that is used to truncate the expansion in Eq. (12.68). These variables and some others are described below.

AIFDEM

AIFDEM
       Perform an AIFDEM calculation.
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       FALSE Do not perform an AIFDEM calculation. TRUE Perform an AIFDEM calculation.
RECOMMENDATION:
       False

AIFDEM_NTOTHRESH

AIFDEM_NTOTHRESH
       Controls how many NTOs that are retained in the exciton-site basis states.
TYPE:
       INTEGER
DEFAULT:
       99
OPTIONS:
       n Retain enough NTOs to recover n% of the norm of the original CIS or TDDFT vectors in Eq. (12.68).
RECOMMENDATION:
       A threshold of 85% gives a good trade-off of computational time and accuracy for organic molecules.

AIFDEM_EMBED_RANGE

AIFDEM_EMBED_RANGE
       Specifies the size of the QM region for charge embedding
TYPE:
       INTEGER
DEFAULT:
       FULL_QM
OPTIONS:
       FULL_QM No charge embedding. 0 Treat only excited fragments with QM. n Range (in Å) from excited fragments within which to treat other fragments with QM.
RECOMMENDATION:
       The minimal threshold of zero typically maintains accuracy while significantly reducing computational time.

AIFDEM_CTSTATES

AIFDEM_CTSTATES
       Include charge-transfer-like cation/anion pair states in the AIFDEM basis.
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       TRUE Include CT states. FALSE Do not include CT states.
RECOMMENDATION:
       Use if CT states are desired in the basis.

AIFDEM_SINGFIS

AIFDEM_SINGFIS
       Include multi-exciton states in the AIFDEM basis.
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       TRUE Include multi-exciton states. FALSE Do not include multi-exciton states.
RECOMMENDATION:
       Use if multi-exciton states are desired in the basis. This option requires the use of AIFDEM_SEGSTART and AIFDEM_SEGEND in the $rem section.

For calculations on large systems, it may be desirable to break up the calculation of the AIFDEM matrix elements into batches. The user can specify can specify the first and last matrix element for a calculation with the following variables.

AIFDEM_SEGSTART

AIFDEM_SEGSTART
       Indicates the index of the first matrix element to be computed.
TYPE:
       INTEGER
DEFAULT:
       NONE
OPTIONS:
       n First matrix element of the chunk to be computed.
RECOMMENDATION:
       Needs to be used with AIFDEM_SEGEND

AIFDEM_SEGEND

AIFDEM_SEGEND
       Indicates the index of the last matrix element to be computed.
TYPE:
       INTEGER
DEFAULT:
       NONE
OPTIONS:
       n Last matrix element of thhe chunk to be computed.
RECOMMENDATION:
       Needs to be used with AIFDEM_SEGSTART

When computing AIFDEM matrix elements in batches, one can avoid running fragment SCF calculations for each job, by first running a job that runs only the fragment SCF calculations. These are saved in the scratch directory. These may then be read into subsequent calculations. The following variables control this utility.

AIFDEM_FRGM_WRITE

AIFDEM_FRGM_WRITE
       Fragment SCF calculations only.
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       TRUE Only fragment SCF calculations are carried out, no computation of matrix elements. FALSE Regular AIFDEM calculation as specified by other $rem variables.
RECOMMENDATION:
       None

AIFDEM_FRGM_READ

AIFDEM_FRGM_READ
       Skips fragment SCF calculations.
TYPE:
       LOGICAL
DEFAULT:
       FALSE
OPTIONS:
       TRUE Skips fragment SCF calculations, only computation of matrix elements. FALSE Regular AIFDEM calculation as specified by other $rem variables.
RECOMMENDATION:
       Requires a prior calculation that computes fragment SCF data.