13.16 Ab Initio Frenkel Davydov Exciton Model (AIFDEM)

13.16.1 Job Control and Examples

A basic AIFDEM calculation is requested by setting AIFDEM = TRUE in the $rem section, with additional job-control variables as described below.

A basic AIFDEM calculation is requested by setting AIFDEM = TRUE in the $rem section, with additional job-control variables as described below.

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
       Controls the number of NTOs that are retained in the exciton-site basis states.
TYPE:
       INTEGER
DEFAULT:
       99
OPTIONS:
       n Threshold percentage of the norm of fragment NTO amplitudes.
RECOMMENDATION:
       A threshold of 85% gives a good trade-off of computational time and accuracy for organic molecules.

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 Å typically maintains accuracy while significantly reducing computational time.

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:
       None

For charge-embedded AIFDEM calculations, set XPOL = TRUE in the $rem section and then select the type of embedding charges via the $xpol input section, as described in Section 13.11 and illustrated in the following example.

Example 13.33  Example showing singlet excited state calculation, on (H2O)4. XPol is used to generate monomer wave functions with ChElPG charges. Minimal QM charge embedding is used for the exciton model with three excited states per fragment.

$molecule
0 1
--H2O 0
   0 1
   O          1.74078        1.59716       -1.49814
   H          2.22908        2.18316       -2.08914
   H          0.88038        2.04726       -1.32684
--H2O 1
   0 1
   O          1.31998       -1.18934       -1.91734
   H          1.49988       -0.22974       -1.89044
   H          1.69058       -1.52594       -1.07704
--H2O 2
   0 1
   O         -0.68982        2.59476       -0.72224
   H         -1.14372        3.37086       -1.07364
   H         -1.35592        1.84986       -0.78334
--H2O 3
   0 1
   O         -1.27512       -1.77394       -1.69524
   H         -0.32252       -1.52884       -1.85604
   H         -1.53992       -2.30454       -2.45644
$end

$rem
   BASIS                aug-cc-pvdz
   EXCHANGE             HF
   CIS_N_ROOTS          3
   CIS_TRIPLETS         FALSE
   XPOL                 TRUE
   AIFDEM               TRUE
   AIFDEM_EMBED_RANGE   0
   AIFDEM_NTOTHRESH     90
   NTO_PAIRS            1
$end

$xpol
  embed   charges
  charges CHELPG
$end

To compute AIFDEM derivatives 𝐇[x] and 𝐒[x] of the Hamiltonian and overlap matrices, the user should request a standard AIFDEM job and in addition set CIS_STATE_DERIV = 1. Currently, the AIFDEM derivatives do not support charge embedding so the keyword AIFDEM_EMBED_RANGE must be omitted from these jobs, which precludes the use of XPol wavefunctions for the fragments. Furthermore, only one excited state per fragment is supported so the keyword CIS_N_ROOTS = 1. is required.

The derivatives of the AIFDEM Hamiltonian matrix and overlap matrix are printed in the output file in sets of the three Cartesian coordinates that belong to a single atom. For convenience, the orthogonalized AIFDEM Hamiltonian matrix elements are saved in the scratch directory, $QCSCRATCH/aifdem_deriv. These are organized such that the derivatives for each unique matrix element are stored in individual files in the order of the atomic Cartesian coordinates. These files can facilitate external calculation of exciton/phonon coupling constants.

Example 13.34  A basic AIFDEM derivative calculation on a chain of helium atoms.

$molecule
0 1
--frgm 0
0 1
He    0.000   0.000   0.000
He    0.000   0.000   1.400
--frgm 1
0 1
He    0.000   0.000   2.800
He    0.000   0.000   4.200
$end

$rem
   BASIS             =  cc-pvdz
   EXCHANGE          =  hf
   AIFDEM            =  true
   CIS_N_ROOTS       =  1
   CIS_SINGLETS      =  true
   CIS_TRIPLETS      =  false
   CIS_STATE_DERIV   =  1
   NTO_PAIRS         =  1
   MEM_TOTAL         =  1000
   MEM_STATIC        =  1000
   MAX_CIS_CYCLES    =  200
   MAX_SCF_CYCLES    =  200
   THRESH            =  10
   AIFDEM_NTOTHRESH  =  100
$end