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

*$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 12.12 and illustrated
in the following example.

$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 ${\mathbf{H}}^{[x]}$ and ${\mathbf{S}}^{[x]}$ 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.

$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