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12.19 The ALMO-CIS/TDA method and its Charge-Transfer Correction

12.19.3 ALMO-CIS/TDA with selected fragment occupied-virtual pairs

(July 14, 2022)

Q-Chem 6.0 and later versions support ALMO-CIS/TDA calculations with selected fragment occupied-virtual pairs, i.e., only excitation amplitudes that correspond to transitions between selected occupied and virtual orbitals are considered in Eq. (12.73). To run this type of calculations one needs to set ALMOCIS_FRAGOV >0, and currently three different modes are supported:

ALMOCIS_FRAGOV

ALMOCIS_FRAGOV
       Doing ALMO-CIS/TDA calculations with transitions from occupied orbitals on the 1st fragment and virtuals in the full system
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Doing standard ALMO-CIS/TDA calculations (if LOCAL_CIS >0) 1 Reading user-specified active fragment O-V pairs from the $frag_ov_pairs section 2 Excitations on the first fragment only 3 Excitations from the occupied orbitals on the first fragment to all virtuals in the system
RECOMMENDATION:
       None

The format of the $frag_ov_pairs section:

$frag_ov_pairs
   [number of frag_ov_pairs]
   [occ_frg_idx1]  [vir_frg_idx1]
   [occ_frg_idx2]  [vir_frg_idx2]
   ...
$end

These modified ALMO-CIS/TDA models can be used to model excited states in complex environments, such as the local excitation of a chromophore in solution or its charge-transfer-to-solvent (CTTS) excitations. Note that the iterative Davidson algorithm is required for these calculations, i.e., EIGSLV_METH = 1.

Example 12.47  ALMO-TDA calculation for foramide water with user-specified occupied-virtual pairs: O(1) -> V(1) and O(1) -> V(2)

$molecule
0 1
--
0 1
  C         1.1508059365    0.2982718924    0.0240277739
  O         0.3545181649    1.2334803420   -0.0015882208
  N         0.8104369587   -1.0072797234    0.0043506838
  H         2.2327270535    0.4686363261    0.0666232655
  H        -0.1675092286   -1.2596328526   -0.0352400180
  H         1.5210524537   -1.7122494331    0.0139809901
--
0 1
  O        -1.9693273428   -0.2999882700   -0.2293071572
  H        -1.3827632725    0.4697313642   -0.1375254289
  H        -2.7470364523   -0.0962178118    0.2907490329
$end

$rem
jobtype     sp
basis       6-31G*
method      pbe0
sym_ignore  true
symmetry    false
frgm_method stoll
cis_n_roots     4
thresh         12
local_cis       1
almocis_fragov  1
eigslv_meth     1  ! iterative method
$end

$frag_ov_pairs
2
1 1
1 2
$end