For electron transfer (ET) and excitation energy transfer (EET) processes, the electronic coupling is one of the important parameters that determine their reaction rates. For ET, Q-Chem provides the coupling values calculated with the generalized Mulliken-Hush (GMH),^{154} fragment-charge difference (FCD),^{945} Boys localization,^{887} and Edmiston-Ruedenbeg^{883} localization schemes. For EET, options include fragment-excitation difference (FED),^{396} fragment-spin difference (FSD),^{1026} occupied-virtual separated Boys localization,^{886} or Edmiston-Ruedenberg localization.^{883} In all these schemes, a vertical excitation such as CIS, RPA or TDDFT is required, and the GMH, FCD, FED, FSD, Boys or ER coupling values are calculated based on the excited state results.
Under the two-state approximation, the diabatic reactant and product states are assumed to be a linear combination of the eigenstates. For ET, the choice of such linear combination is determined by a zero transition dipoles (GMH) or maximum charge differences (FCD). In the latter, a $2\times 2$ donor–acceptor charge difference matrix, $\mathrm{\Delta}\mathbf{q}$, is defined, with elements
$$\mathrm{\Delta}{q}_{mn}={q}_{mn}^{\text{D}}-{q}_{mn}^{\text{A}}={\int}_{\mathbf{r}\in \text{D}}{\rho}_{mn}(\mathbf{r})\mathit{d}\mathbf{r}-{\int}_{\mathbf{r}\in \text{A}}{\rho}_{mn}(\mathbf{r})\mathit{d}\mathbf{r}$$ |
where ${\rho}_{mn}(\mathbf{r})$ is the matrix element of the density operator between states $|m\u27e9$ and $|n\u27e9$.
For EET, a maximum excitation difference is assumed in the FED, in which a excitation difference matrix is similarly defined with elements
$$\mathrm{\Delta}{x}_{mn}={x}_{mn}^{\text{D}}-{x}_{mn}^{\text{A}}={\int}_{\mathbf{r}\in \text{D}}{\rho}_{\mathrm{ex}}^{(mn)}(\mathbf{r})\mathit{d}\mathbf{r}-{\int}_{\mathbf{r}\in \text{A}}{\rho}_{\mathrm{ex}}^{(mn)}(\mathbf{r})\mathit{d}\mathbf{r}$$ |
where ${\rho}_{\mathrm{ex}}^{(mn)}(\mathbf{r})$ is the sum of attachment and detachment densities for transition $|m\u27e9\to |n\u27e9$, as they correspond to the electron and hole densities in an excitation. In the FSD, a maximum spin difference is used and the corresponding spin difference matrix is defined with its elements as,
$$\mathrm{\Delta}{s}_{mn}={s}_{mn}^{\text{D}}-{s}_{mn}^{\text{A}}={\int}_{\mathbf{r}\in \text{D}}{\sigma}_{(mn)}(\mathbf{r})\mathit{d}\mathbf{r}-{\int}_{\mathbf{r}\in \text{A}}{\sigma}_{(mn)}(\mathbf{r})\mathit{d}\mathbf{r}$$ |
where ${\sigma}_{mn}(\mathbf{r})$ is the spin density, difference between $\alpha $-spin and $\beta $-spin densities, for transition from $|m\u27e9\to |n\u27e9$.
Since Q-Chem uses a Mulliken population analysis for the integrations in Eqs. (11.16.1.1), (11.16.1.1), and (11.16.1.1), the matrices $\mathrm{\Delta}\mathbf{q}$, $\mathrm{\Delta}\mathbf{x}$ and $\mathrm{\Delta}\mathbf{s}$ are not symmetric. To obtain a pair of orthogonal states as the diabatic reactant and product states, $\mathrm{\Delta}\mathbf{q}$, $\mathrm{\Delta}\mathbf{x}$ and $\mathrm{\Delta}\mathbf{s}$ are symmetrized in Q-Chem. Specifically,
${\overline{\mathrm{\Delta}q}}_{mn}$ | $=(\mathrm{\Delta}{q}_{mn}+\mathrm{\Delta}{q}_{nm})/2$ | (11.84a) | ||
${\overline{\mathrm{\Delta}x}}_{mn}$ | $=(\mathrm{\Delta}{x}_{mn}+\mathrm{\Delta}{x}_{nm})/2$ | (11.84b) | ||
${\overline{\mathrm{\Delta}s}}_{mn}$ | $=(\mathrm{\Delta}{s}_{mn}+\mathrm{\Delta}{s}_{nm})/2$ | (11.84c) |
The final coupling values are obtained as listed below:
For GMH,
$${V}_{\text{ET}}=\frac{({E}_{2}-{E}_{1})\left|{\overrightarrow{\mu}}_{12}\right|}{\sqrt{{({\overrightarrow{\mu}}_{11}-{\overrightarrow{\mu}}_{22})}^{2}+4{\left|{\overrightarrow{\mu}}_{12}\right|}^{2}}}$$ | (11.85) |
For FCD,
$${V}_{\text{ET}}=\frac{({E}_{2}-{E}_{1}){\overline{\mathrm{\Delta}q}}_{12}}{\sqrt{{(\mathrm{\Delta}{q}_{11}-\mathrm{\Delta}{q}_{22})}^{2}+4{\overline{\mathrm{\Delta}q}}_{12}^{2}}}$$ | (11.86) |
For FED,
$${V}_{\text{EET}}=\frac{({E}_{2}-{E}_{1}){\overline{\mathrm{\Delta}x}}_{12}}{\sqrt{{(\mathrm{\Delta}{x}_{11}-\mathrm{\Delta}{x}_{22})}^{2}+4{\overline{\mathrm{\Delta}x}}_{12}^{2}}}$$ | (11.87) |
For FSD,
$${V}_{\text{EET}}=\frac{({E}_{2}-{E}_{1}){\overline{\mathrm{\Delta}s}}_{12}}{\sqrt{{(\mathrm{\Delta}{s}_{11}-\mathrm{\Delta}{s}_{22})}^{2}+4{\overline{\mathrm{\Delta}s}}_{12}^{2}}}$$ | (11.88) |
Q-Chem provides the option to control FED, FSD, FCD and GMH calculations after a single-excitation calculation, such as CIS, RPA, TDDFT/TDA and TDDFT. To obtain ET coupling values using GMH (FCD) scheme, one should set $rem variables STS_GMH (STS_FCD) to be TRUE. Similarly, a FED (FSD) calculation is turned on by setting the $rem variable STS_FED (STS_FSD) to be TRUE. In FCD, FED and FSD calculations, the donor and acceptor fragments are defined via the $rem variables STS_DONOR and STS_ACCEPTOR. It is necessary to arrange the atomic order in the $molecule section such that the atoms in the donor (acceptor) fragment is in one consecutive block. The ordering numbers of beginning and ending atoms for the donor and acceptor blocks are included in $rem variables STS_DONOR and STS_ACCEPTOR.
The couplings will be calculated between all choices of excited states with the same spin. In FSD, FCD and GMH calculations, the coupling value between the excited and reference (ground) states will be included, but in FED, the ground state is not included in the analysis. It is important to select excited states properly, according to the distribution of charge or excitation, among other characteristics, such that the coupling obtained can properly describe the electronic coupling of the corresponding process in the two-state approximation.
STS_GMH
Control the calculation of GMH for ET couplings.
TYPE:
LOGICAL
DEFAULT:
FALSE
OPTIONS:
FALSE
Do not perform a GMH calculation.
TRUE
Include a GMH calculation.
RECOMMENDATION:
When set to true computes Mulliken-Hush electronic couplings. It yields
the generalized Mulliken-Hush couplings as well as the transition dipole
moments for each pair of excited states and for each excited state with
the ground state.
STS_FCD
Control the calculation of FCD for ET couplings.
TYPE:
LOGICAL
DEFAULT:
FALSE
OPTIONS:
FALSE
Do not perform an FCD calculation.
TRUE
Include an FCD calculation.
RECOMMENDATION:
None
STS_FED
Control the calculation of FED for EET couplings.
TYPE:
LOGICAL
DEFAULT:
FALSE
OPTIONS:
FALSE
Do not perform a FED calculation.
TRUE
Include a FED calculation.
RECOMMENDATION:
None
STS_FSD
Control the calculation of FSD for EET couplings.
TYPE:
LOGICAL
DEFAULT:
FALSE
OPTIONS:
FALSE
Do not perform a FSD calculation.
TRUE
Include a FSD calculation.
RECOMMENDATION:
For RCIS triplets, FSD and FED are equivalent. FSD will be automatically
switched off and perform a FED calculation.
STS_DONOR
Define the donor fragment.
TYPE:
STRING
DEFAULT:
0
No donor fragment is defined.
OPTIONS:
$i$-$j$
Donor fragment is in the $i$th atom to the $j$th atom.
RECOMMENDATION:
Note no space between the hyphen and the numbers $i$ and $j$.
STS_ACCEPTOR
Define the acceptor molecular fragment.
TYPE:
STRING
DEFAULT:
0
No acceptor fragment is defined.
OPTIONS:
$i$-$j$
Acceptor fragment is in the $i$th atom to the $j$th atom.
RECOMMENDATION:
Note no space between the hyphen and the numbers $i$ and $j$.
STS_MOM
Control calculation of the transition moments between excited states in
the CIS and TDDFT calculations (including SF variants).
TYPE:
LOGICAL
DEFAULT:
FALSE
OPTIONS:
FALSE
Do not calculate state-to-state transition moments.
TRUE
Do calculate state-to-state transition moments.
RECOMMENDATION:
When set to true requests the state-to-state dipole transition moments for
all pairs of excited states and for each excited state with the ground
state.
$molecule 1 1 C 0.679952 0.000000 0.000000 N -0.600337 0.000000 0.000000 H 1.210416 0.940723 0.000000 H 1.210416 -0.940723 0.000000 H -1.131897 -0.866630 0.000000 H -1.131897 0.866630 0.000000 C -5.600337 0.000000 0.000000 C -6.937337 0.000000 0.000000 H -5.034682 0.927055 0.000000 H -5.034682 -0.927055 0.000000 H -7.502992 -0.927055 0.000000 H -7.502992 0.927055 0.000000 $end $rem METHOD CIS BASIS 6-31+G CIS_N_ROOTS 20 CIS_SINGLETS true CIS_TRIPLETS false STS_GMH true !turns on the GMH calculation STS_FCD true !turns on the FCD calculation STS_DONOR 1-6 !define the donor fragment as atoms 1-6 for FCD calc. STS_ACCEPTOR 7-12 !define the acceptor fragment as atoms 7-12 for FCD calc. MEM_STATIC 200 !increase static memory for a CIS job with larger basis set $end
$molecule 0 1 C 0.670518 0.000000 0.000000 H 1.241372 0.927754 0.000000 H 1.241372 -0.927754 0.000000 C -0.670518 0.000000 0.000000 H -1.241372 -0.927754 0.000000 H -1.241372 0.927754 0.000000 C 0.774635 0.000000 4.500000 H 1.323105 0.936763 4.500000 H 1.323105 -0.936763 4.500000 C -0.774635 0.000000 4.500000 H -1.323105 -0.936763 4.500000 H -1.323105 0.936763 4.500000 $end $rem METHOD CIS BASIS 3-21G CIS_N_ROOTS 20 CIS_SINGLETS true CIS_TRIPLETS false STS_FED true STS_DONOR 1-6 STS_ACCEPTOR 7-12 $end
When dealing with multiple charge or electronic excitation centers, diabatic states can be constructed with Boys^{887} or Edmiston-Ruedenberg^{883} localization. In this case, we construct diabatic states $\left\{|{\mathrm{\Xi}}_{I}\u27e9\right\}$ as linear combinations of adiabatic states $\left\{|{\mathrm{\Phi}}_{I}\u27e9\right\}$ with a general rotation matrix $\mathbf{U}$ that is ${N}_{\mathrm{state}}\times {N}_{\mathrm{state}}$ in size:
$$|{\mathrm{\Xi}}_{I}\u27e9=\sum _{J=1}^{{N}_{\mathrm{states}}}|{\mathrm{\Phi}}_{J}\u27e9{U}_{ji}\mathit{\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}}I=1\mathrm{\dots}{N}_{\mathrm{states}}$$ | (11.89) |
The adiabatic states can be produced with any method, in principle, but the Boys/ER-localized diabatization methods have been implemented thus far only for CIS or TDDFT methods in Q-Chem. In analogy to orbital localization, Boys-localized diabatization corresponds to maximizing the charge separation between diabatic state centers:
$${f}_{\mathrm{Boys}}(\mathbf{U})={f}_{\mathrm{Boys}}(\{{\mathrm{\Xi}}_{I}\})=\sum _{I,J=1}^{{N}_{\mathrm{states}}}|\u27e8{\mathrm{\Xi}}_{I}|\overrightarrow{\mu}|{\mathrm{\Xi}}_{I}\u27e9-\u27e8{\mathrm{\Xi}}_{J}|\overrightarrow{\mu}|{\mathrm{\Xi}}_{J}\u27e9{|}^{2}$$ | (11.90) |
Here, $\overrightarrow{\mu}$ represents the dipole operator. ER-localized diabatization prescribes maximizing self-interaction energy:
$${f}_{ER}(\mathbf{U})={f}_{\mathrm{ER}}(\left\{{\mathrm{\Xi}}_{I}\right\})=\sum _{I=1}^{{N}_{\mathrm{states}}}\int \mathit{d}{\overrightarrow{\mathcal{R}}}_{1}\int \mathit{d}{\overrightarrow{\mathcal{R}}}_{2}\frac{\u27e8{\mathrm{\Xi}}_{I}|\widehat{\rho}({\overrightarrow{\mathcal{R}}}_{2})|{\mathrm{\Xi}}_{I}\u27e9\u27e8{\mathrm{\Xi}}_{I}|\widehat{\rho}({\overrightarrow{\mathcal{R}}}_{1})|{\mathrm{\Xi}}_{I}\u27e9}{|{\overrightarrow{\mathcal{R}}}_{1}-{\overrightarrow{\mathcal{R}}}_{2}|}$$ | (11.91) |
where the density operator at position $\overrightarrow{\mathcal{R}}$ is
$$\widehat{\rho}(\overrightarrow{\mathcal{R}})=\sum _{j}\delta (\overrightarrow{\mathcal{R}}-{\overrightarrow{r}}^{(j)})$$ | (11.92) |
Here, ${\overrightarrow{r}}^{(j)}$ represents the position of the $j$th electron.
These models reflect different assumptions about the interaction of our quantum system with some fictitious external electric field/potential: $(i)$ if we assume a fictitious field that is linear in space, we arrive at Boys localization; $(ii)$ if we assume a fictitious potential energy that responds linearly to the charge density of our system, we arrive at ER localization. Note that in the two-state limit, Boys localized diabatization reduces nearly exactly to GMH.^{887}
As written down in Eq. (11.90), Boys localized diabatization applies only to charge transfer, not to energy transfer. Within the context of CIS or TDDFT calculations, one can easily extend Boys localized diabatization^{886} by separately localizing the occupied and virtual components of $\overrightarrow{\mu}$, ${\overrightarrow{\mu}}^{\mathrm{occ}}$ and ${\overrightarrow{\mu}}^{\mathrm{virt}}$:
$\begin{array}{cc}\hfill {f}_{\mathrm{BoysOV}}(\mathbf{U})& ={f}_{\mathrm{BoysOV}}(\{{\mathrm{\Xi}}_{I}\})\hfill \\ & ={\displaystyle \sum _{I,J=1}^{{N}_{\mathrm{states}}}}(|\u27e8{\mathrm{\Xi}}_{I}|{\overrightarrow{\mu}}^{\mathrm{occ}}|{\mathrm{\Xi}}_{I}\u27e9-\u27e8{\mathrm{\Xi}}_{J}|{\overrightarrow{\mu}}^{\mathrm{occ}}|{\mathrm{\Xi}}_{J}\u27e9{|}^{2}+{|\u27e8{\mathrm{\Xi}}_{I}|{\overrightarrow{\mu}}^{\mathrm{virt}}|{\mathrm{\Xi}}_{I}\u27e9-\u27e8{\mathrm{\Xi}}_{J}|{\overrightarrow{\mu}}^{\mathrm{virt}}|{\mathrm{\Xi}}_{J}\u27e9|}^{2})\hfill \end{array}$ | (11.93) |
where
$$|{\mathrm{\Xi}}_{I}\u27e9=\sum _{ia}{t}_{i}^{Ia}|{\mathrm{\Phi}}_{i}^{a}\u27e9$$ | (11.94) |
and the occupied/virtual components are defined by
$$ | $=$ | $\underset{\u23df}{{\delta}_{IJ}{\displaystyle \sum _{i}}{\overrightarrow{\mu}}_{ii}-{\displaystyle \sum _{aij}}{t}_{i}^{Ia}{t}_{j}^{Ja}{\overrightarrow{\mu}}_{ij}}+\underset{\u23df}{{\displaystyle \sum _{iba}}{t}_{i}^{Ia}{t}_{i}^{Jb}{\overrightarrow{\mu}}_{ab}}$ | ||
$$ |
Note that when we maximize the Boys OV function, we are simply performing Boys-localized diabatization separately on the electron attachment and detachment densities.
Finally, for energy transfer, it can be helpful to understand the origin of the diabatic couplings. To that end, we now provide the ability to decompose the diabatic coupling between diabatic states into Coulomb (J), Exchange (K) and one-electron (O) components:^{949}
$$ | $=$ | $\underset{\u23df}{{\displaystyle \sum _{iab}}{t}_{i}^{Pa}{t}_{i}^{Qb}{F}_{ab}-{\displaystyle \sum _{ija}}{t}_{i}^{Pa}{t}_{j}^{Qa}{F}_{ij}}+\underset{\u23df}{{\displaystyle \sum _{ijab}}{t}_{i}^{Pa}{t}_{j}^{Qb}\left(ia|jb\right)}-\underset{\u23df}{{\displaystyle \sum _{ijab}}{t}_{i}^{Pa}{t}_{j}^{Qb}\left(ij|ab\right)}$ | (11.96) | ||
$\mathrm{\hspace{0.33em}}\mathit{\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{0.5em}\hspace{1em}}O\mathit{\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{0.25em}}J\mathit{\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}}K$ |
BOYS_CIS_NUMSTATE
Define how many states to mix with Boys localized diabatization. These states must be specified
in the $localized_diabatization section.
TYPE:
INTEGER
DEFAULT:
0
Do not perform Boys localized diabatization.
OPTIONS:
2 to N where N is the number of CIS states requested (CIS_N_ROOTS)
RECOMMENDATION:
It is usually not wise to mix adiabatic states that are separated by more than a few eV
or a typical reorganization energy in solvent.
ER_CIS_NUMSTATE
Define how many states to mix with ER localized diabatization. These states must be specified
in the $localized_diabatization section.
TYPE:
INTEGER
DEFAULT:
0
Do not perform ER localized diabatization.
OPTIONS:
2 to N where N is the number of CIS states requested (CIS_N_ROOTS)
RECOMMENDATION:
It is usually not wise to mix adiabatic states that are separated by more than a few eV
or a typical reorganization energy in solvent.
LOC_CIS_OV_SEPARATE
Decide whether or not to localized the “occupied” and “virtual” components
of the localized diabatization
function, i.e., whether to localize the electron attachments and detachments separately.
TYPE:
LOGICAL
DEFAULT:
FALSE
Do not separately localize electron attachments and detachments.
OPTIONS:
TRUE
RECOMMENDATION:
If one wants to use Boys localized diabatization for energy transfer (as
opposed to electron transfer) , this is a necessary option. ER is more
rigorous technique, and does not require this OV feature, but will be somewhat
slower.
CIS_DIABATH_DECOMPOSE
Decide whether or not to decompose the diabatic coupling into Coulomb,
exchange, and one-electron terms.
TYPE:
LOGICAL
DEFAULT:
FALSE
Do not decompose the diabatic coupling.
OPTIONS:
TRUE
RECOMMENDATION:
These decompositions are most meaningful for electronic excitation transfer processes.
Currently, available only for CIS, not for TDDFT diabatic states.
$molecule 0 1 he 0 -1.0 1.0 he 0 -1.0 -1.0 he 0 1.0 -1.0 he 0 1.0 1.0 $end $rem METHOD cis CIS_N_ROOTS 4 CIS_SINGLETS false CIS_TRIPLETS true BASIS 6-31g** SCF_CONVERGENCE 8 SYMMETRY false RPA false SYM_IGNORE true LOC_CIS_OV_SEPARATE false ! NOT localizing attachments/detachments separately. ER_CIS_NUMSTATE 4 ! using ER to mix 4 adiabatic states. CIS_DIABATh_DECOMPOSE true ! decompose diabatic couplings into ! Coulomb, exchange, and one-electron components. $end $localized_diabatization On the next line, list which excited adiabatic states we want to mix. 1 2 3 4 $end