The ALMO-EDA for intermolecular interactions involving excited-state molecules implemented in Q-Chem 5.1 supports CIS and TDDFT within the Tamm-Dancoff approximation (TDA) for closed-shell systems, i.e., excited states calculated by TDDFT and unrestricted systems are currently not supported. The EDA procedure is triggered by setting EX_EDA to TRUE. The code first performs a customized ground-state calculation (using AO-based ALMOs) through the “EDA2" driver. During the isolated fragment calculations in this EDA, the fragment excited states are also computed after its ground-state SCF is converged, which is controlled by a new input section $frgm_cis_n_roots. The format of this input section is as follows:
$frgm_cis_n_roots ¯frgm_idx1¯¯nstates_to_calc¯¯nstates_as_exciton_basis ¯frgm_idx2¯¯nstates_to_calc¯¯nstates_as_exciton_basis ¯. . . $end
Here “nstates_to_calc" specifies the number of states to calculate for each fragment (the value of CIS_N_ROOTS for each fragment calculation), and “nstates_as_exciton_basis" refers to the number of calculated fragment states that are used to construct the EXSP state (whose sum gives in eqn. 13.26). When the supersystem is considered as an exciplex where the excitation is assigned to a specific fragment, only one row is needed in this section, and there is no need to specify the number of states used as the basis for the EXSP state.
The other relevant rem variables includes CIS_N_ROOTS, which specifies the number of roots to calculate in the ALMO-CIS/TDA and full CIS/TDA calculations, and EIGSLV_METH (see Sec. 13.18) that is set to 1 (using the Davidson iterative solver) by default. Note that the number of states that the EDA is concerned with is controlled by the number of isolated fragment states (the exciplex case) or the total number of states that are excitonically coupled (the excimer case). In the latter case, CIS_N_ROOTS is usually set to a value that is larger than to ensure that all states of interest are captured in the ALMO-CIS/TDA and full CIS/TDA calculations, as changes in state-ordering might occur.
$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 eda ex_eda true method hf basis 6-31+g(d) sym_ignore true symmetry false scf_convergence 8 cis_n_roots 2 cis_triplets false thresh 12 $end $frgm_cis_n_roots 1 2 $end
$molecule 0 1 -- 0 1 He 0.0 0.0 0.0 -- 0 1 He 3.0 0.0 0.0 $end $rem JOBTYPE eda EX_EDA true METHOD hf BASIS gen !6-311(2+,2+)G SYM_IGNORE true SYMMETRY false CIS_N_ROOTS 8 CIS_TRIPLETS false THRESH 12 EIGSLV_METH 0 !direct $end $frgm_cis_n_roots 1 8 1 2 8 1 $end $basis He 0 S 3 1.000000 9.81243000E+01 2.87452000E-02 1.47689000E+01 2.08061000E-01 3.31883000E+00 8.37635000E-01 S 1 1.000000 8.74047000E-01 1.00000000E+00 S 1 1.000000 2.44564000E-01 1.00000000E+00 SP 1 1.000000 4.80000000E-02 1.00000000E+00 1.00000000E+00 SP 1 1.000000 1.44578313E-02 1.00000000E+00 1.00000000E+00 **** $end