12.6.5 Job Control for EDA2

The use of the FERF model for the evaluation of polarization energy and the further decomposition of the frozen term define the second generation of the ALMO-EDA method. Meanwhile, under the same code structure, the original AO-block based ALMO model and other related methods (such as the constrained relaxation of the frozen wave function ⁵⁵⁷ Horn P. R., Head-Gordon M.
J. Chem. Phys.
(2016), 144, pp. 084118. Link which renders the frozen energy variationally computed, and the polMO model ⁵⁴ Azar R. J. et al.
J. Chem. Phys.
(2013), 138, pp. 084102. Link that arguably gives a lower limit to the polarization contribution) are also available. This entire set of methods implemented in Q-Chem based on GEN_SCFMAN (see Section 4.3) is referred to as “EDA2". In Q-Chem 5.2 and after, “EDA2" is used as the default ALMO-EDA driver when “JOBTYPE = EDA" is requested.

The job control for EDA2 is largely simplified by a series of preset options provided by the developers. The option number is set through the EDA2 $rem variable (introduced below). Besides that, for the sake of flexibility, users are allowed to overwrite the values of part of the preset $rem variables:

• •

  Related to the isolated fragment calculations:

  • –

    EDA_CHILD_SUPER_BASIS: use the super-system basis for fragment calculations (default: FALSE).

  • –

    FRAGMO_GUESS_MODE: as introduced in Section 12.3 (default: 0).

• •

  Related to the decomposition of the FRZ term:

  • –

    FRZ_ORTHO_DECOMP: it can be turned off by setting its value to $-1$ in the $rem section
    (default: TRUE).

  • –

    FRZ_ORTHO_DECOMP_CONV: as introduced in Section 12.6.4 (default: 6).

  • –

    EDA_CLS_DISP: as introduced in Section 12.6.4 (default: FALSE).

  • –

    DISP_FREE_X: as introduced in Section 12.6.4 (default: HF).

  • –

    DISP_FREE_C: as introduced in Section 12.6.4 (default: NONE).

• •

  Related to the evaluation of POL:

  • –

    CHILD_MP_ORDERS: as introduced in Section 12.6.3 (default: 232 (DQ)).

  • –

    SCFMI_FREEZE_SS: as introduced in Section 12.6.2 (default: 0).

• •

  Related to the evaluation of CT and BSSE:

  • –

    EDA_NO_CT: skip the evaluation of the CT term in the EDA procedure
    (default: FALSE (automatically turned on when SCFMI_FREEZE_SS = TRUE)).

  • –

    EDA_BSSE: use counterpoise-corrected monomer calculations to evaluate the BSSE
    (default: FALSE).

  • –

    EDA_PCT_A: turn on perturbative charge transfer analysis (Roothaan step based).

  • –

    EDA_COVP: perform COVP analysis for charge transfer (see Section 12.5.3).

  • –

    EDA_PRINT_COVP: dump COVPs to the MO coefficient file (see Section 12.5.3). Note: EDA2 can automatically generate the cubes for the dominant complementary occupied-virtual orbitals for each pair of donor and acceptor fragments when EDA_PRINT_COVP is greater than 1.

Note that starting with Q-Chem v. 5.2, if JOBTYPE = EDA is requested while but the $rem variable EDA2 is not specified by the user, the latter defaults to EDA2 = 2 with EDA_PCT_A = TRUE.

For calculations based on EDA2, the default SCF convergence criterion is set to 10${}^{-8}$ (rather than 10${}^{-5}$ as in Q-Chem’s normal SCF calculations). The fragment calculations involved are forced to use the same SCF convergence criterion as the parent job.

In cases where the radical is a single atom (e.g. $\cdot$Cl) or of a highly symmetric geometry (e.g. ${}^{\bullet }$OH), there can be multiple degenerate electronic configurations with the unpaired electron residing in different orbitals, resulting in arbitrariness in the definition of the frozen state. For such systems, it is desirable to obtain the orientation of fragment spin that leads to the lowest-energy frozen state. This can be achieved by employing a “polarize-then-depolarize" (PtD) approach, ⁸³⁷ Mao Y. et al.
Phys. Chem. Chem. Phys.
(2020), 22, pp. 12867. Link using interfragment polarization to resolve the degeneracy of radical’s electronic states: one first converges the polarization (SCF-MI) calculation for the full system, and then recalculates the SCF solutions for each isolated fragment using their corresponding blocks in the ALMO coefficient matrix as the initial guess. To ensure that the “depolarized” fragments are of the same electronic configuration as in the fully polarized wavefunction, the initial maximum overlap method ⁶⁸ Barca G. M. J., Gilbert A. T. B., Gill P. M. W.
J. Chem. Theory Comput.
(2018), 14, pp. 1501. Link (IMOM) is used in these fragment calculations. The fragment orbitals obtained therefrom then uniquely determine the frozen wavefunction.

In Q-Chem 5.2 and after, the procedure described above is performed for unrestricted ALMO-EDA calculations by default. It can also be manually requested by setting EDA_ALIGN_FRGM_SPIN to $>0$ values. Note that this setting further ensures that one obtains a stable SCF-MI solution in the initial polarization step (see below), which is crucial for the success of this approach. Occasionally, one may find that the frozen state constructed from “spin-aligned” fragments is of a higher energy than the initial one. This indicates that the fragment spin alignment procedure is not functioning well, and in such cases we recommend the user to run EDA2 calculation without this procedure by setting FRZ_RELAX = $-1$.

Another feature that can be useful for systems involving open-shell species is the capability of performing stability analysis on user-specified fragments, since it is important to ensure the stability of each fragment’s SCF solution. This can be done through the $frgm_stability input section:

$frgm_stability
   [frgm_idx1]  [frgm_idx2]  ...
$end

where one simply puts the indices of fragments that require stability analysis.