11.16 Electronic Couplings for Electron- and Energy Transfer

11.16.1 Eigenstate-Based Methods

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-Ruedenbeg883 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.

11.16.1.1 Two-state approximation

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 2×2 donor–acceptor charge difference matrix, Δ𝐪, is defined, with elements

Δqmn=qmnD-qmnA=𝐫Dρmn(𝐫)𝑑𝐫-𝐫Aρmn(𝐫)𝑑𝐫

where ρmn(𝐫) is the matrix element of the density operator between states |m and |n.

For EET, a maximum excitation difference is assumed in the FED, in which a excitation difference matrix is similarly defined with elements

Δxmn=xmnD-xmnA=𝐫Dρex(mn)(𝐫)𝑑𝐫-𝐫Aρex(mn)(𝐫)𝑑𝐫

where ρex(mn)(𝐫) is the sum of attachment and detachment densities for transition |m|n, 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,

Δsmn=smnD-smnA=𝐫Dσ(mn)(𝐫)𝑑𝐫-𝐫Aσ(mn)(𝐫)𝑑𝐫

where σmn(𝐫) is the spin density, difference between α-spin and β-spin densities, for transition from |m|n.

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 Δ𝐪, Δ𝐱 and Δ𝐬 are not symmetric. To obtain a pair of orthogonal states as the diabatic reactant and product states, Δ𝐪, Δ𝐱 and Δ𝐬 are symmetrized in Q-Chem. Specifically,

Δq¯mn =(Δqmn+Δqnm)/2 (11.84a)
Δx¯mn =(Δxmn+Δxnm)/2 (11.84b)
Δs¯mn =(Δsmn+Δsnm)/2 (11.84c)

The final coupling values are obtained as listed below:

  • For GMH,

    VET=(E2-E1)|μ12|(μ11-μ22)2+4|μ12|2 (11.85)
  • For FCD,

    VET=(E2-E1)Δq¯12(Δq11-Δq22)2+4Δq¯122 (11.86)
  • For FED,

    VEET=(E2-E1)Δx¯12(Δx11-Δx22)2+4Δx¯122 (11.87)
  • For FSD,

    VEET=(E2-E1)Δs¯12(Δs11-Δs22)2+4Δs¯122 (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 ith atom to the jth 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 ith atom to the jth 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.

Example 11.32  A GMH & FCD calculation to analyze electron-transfer couplings in an ethylene and a methaniminium cation.

$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

Example 11.33  An FED calculation to analyze excitation-energy transfer couplings in a pair of stacked ethylenes.

$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

11.16.1.2 Multi-state treatments

When dealing with multiple charge or electronic excitation centers, diabatic states can be constructed with Boys887 or Edmiston-Ruedenberg883 localization. In this case, we construct diabatic states {|ΞI} as linear combinations of adiabatic states {|ΦI} with a general rotation matrix 𝐔 that is Nstate×Nstate in size:

|ΞI=J=1Nstates|ΦJUji    I=1Nstates (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:

fBoys(𝐔)=fBoys({ΞI})=I,J=1Nstates|ΞI|μ|ΞI-ΞJ|μ|ΞJ|2 (11.90)

Here, μ represents the dipole operator. ER-localized diabatization prescribes maximizing self-interaction energy:

fER(𝐔)=fER({ΞI})=I=1Nstates𝑑1𝑑2ΞI|ρ^(2)|ΞIΞI|ρ^(1)|ΞI|1-2| (11.91)

where the density operator at position is

ρ^()=jδ(-r(j)) (11.92)

Here, r(j) represents the position of the jth 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 diabatization886 by separately localizing the occupied and virtual components of μ, μocc and μvirt:

fBoysOV(𝐔)=fBoysOV({ΞI})=I,J=1Nstates(|ΞI|μocc|ΞI-ΞJ|μocc|ΞJ|2+|ΞI|μvirt|ΞI-ΞJ|μvirt|ΞJ|2) (11.93)

where

|ΞI=iatiIa|Φia (11.94)

and the occupied/virtual components are defined by

ΞI|μ|ΞJ = δIJiμii-aijtiIatjJaμij+ibatiIatiJbμab
   ΞI|μocc|ΞJ   +ΞI|μvirt|ΞJ

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

ΞP|H|ΞQ = iabtiPatiQbFab-ijatiPatjQaFij+ijabtiPatjQb(ia|jb)-ijabtiPatjQb(ij|ab) (11.96)
       O           J        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.

Example 11.34  A calculation using ER localized diabatization to construct the diabatic Hamiltonian and couplings between a square of singly-excited Helium atoms.

$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