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# 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