# Electron Transfer
The NWChem electron transfer (ET) module calculates the electronic
coupling energy (also called the electron transfer matrix element)
between ET reactant and product states. The electronic coupling
(VRP), and nuclear
reorganization energy (λ) are all components of the electron transfer
rate defined by Marcus' theory, which also depends on the temperature
(see Reference 1
below):
$${k_{ET}}= \frac{2\pi}{\hbar} V_{RP}^{2} \frac{1}{\sqrt{4\pi \lambda k_{B}T}} \exp \left( \frac{- \Delta G^{*}}{k_{B} T} \right)$$
The ET module utilizes the method of *Corresponding Orbital
Transformation* to calculate VRP. The only input required are the
names of the files containing the open-shell (UHF) MO vectors for the ET
reactant and product states (*R* and *P*).
The basis set used in the calculation of VRP must be the same as
the basis set used to calculate the MO vectors of *R* and *P*. The
magnitude of VRP depends on the amount of overlap between *R* and
*P*, which is important to consider when choosing the basis set. Diffuse
functions may be necessary to fill in the overlap, particularly when the
ET distance is long.
The MO's of *R* and *P* must correspond to localized states. for
instance, in the reaction A- + B → A + B- the
transferring electron is localized on A in the reactant state and is
localized on B in the product state. To verify the localization of the
electron in the calculation of the vectors, carefully examine the
Mulliken population analysis. In order to determine which orbitals are
involved in the electron transfer, use the print keyword "mulliken ao"
which prints the Mulliken population of each basis function.
An effective core potential (ECP) basis can be used to replace core
electrons. However, there is one caveat: the orbitals involved in
electron transfer must not be replaced with ECP's. Since the ET orbitals
are valence orbitals, this is not usually a problem, but the user should
use ECP's with care.
Suggested references are listed below. The first two references gives a
good description of Marcus' two-state ET model, and the appendix of the
third reference details the method used in the ET module.
1. R.A. Marcus, N. Sutin, Biochimica Biophysica Acta 35, 437, (1985).
2. J.R. Bolton, N. Mataga, and G. McLendon in "Electron Transfer in
Inorganic, Organic and Biological Systems" (American Chemical
Society, Washington, D.C., 1991)
3. A. Farazdel, M. Dupuis, E. Clementi, and A. Aviram, J. Am. Chem.
Soc., 112, 4206 (1990).
## VECTORS: input of MO vectors for ET reactant and product states
```
VECTORS [reactants]
VECTORS [products ]
```
In the VECTORS directive the user specifies the source of the molecular
orbital vectors for the ET reactant and product states. This is required
input, as no default filename will be set by the program. In fact, this
is the only required input in the ET module, although there are other
optional keywords described
below.
## FOCK/NOFOCK: method for calculating the two-electron contribution to VRP
```
```
This directive enables/disables the use of the NWChem's Fock matrix
routine in the calculation of the two-electron portion of the ET
Hamiltonian. Since the Fock matrix routine has been optimized for speed,
accuracy and parallel performance, it is the most efficient choice.
Alternatively, the user can calculate the two-electron contribution to
the ET Hamiltonian with another subroutine which may be more accurate
for systems with a small number of basis functions, although it is
slower.
## TOL2E: integral screening threshold
```
TOL2E
```
The variable tol2e is used in determining the integral screening
threshold for the evaluation of the two-electron contribution to the
Hamiltonian between the electron transfer reactant and product states.
As a default, tol2e is set depending on the magnitude of the overlap
between the ET reactant and product states (SRP), and is not less
than 1.0d-12 or greater than 1.0d-7.
The input to specify the threshold explicitly within the ET directive
is, for example:
```
tol2e 1e-9
```
## Example
The following example is for a simple electron transfer reaction,
He → He+. The ET calculation is easy to execute,
but it is crucial that ET reactant and product wavefunctions reflect
localized states. This can be accomplished using either a [fragment
guess](Hartree-Fock-Theory-for-Molecules.md#vectors-fragment-superposition-of-fragment-molecular-orbitals), or
a [charged atomic density
guess](Hartree-Fock-Theory-for-Molecules.md#atomic-guess-orbitals-with-charged-atoms). For
self-exchange ET reactions such as this one, you can use the
[REORDER](Hartree-Fock-Theory-for-Molecules.md#vectors-inputoutput-of-mo-vectors) keyword
to move the electron from the first helium to the second.
Example input :
```
basis "ao basis"
* library aug-cc-pvtz
end
geometry
He 0 0 0
end
charge 1
scf
tol2e 1d-9
uhf
doublet
vectors output HeP.movecs
end
task scf
charge 0
scf
uhf
singlet
vectors output He.movecs
end
task scf
geometry noautosym noautoz
He 0.0 0.0 0.0
He 5.0 0.0 0.0
end
charge 1
#ET reactants:
scf
doublet; uhf; vectors input fragment HeP.movecs He.movecs output HeA.movecs
end
task scf
#ET products:
scf
doublet; uhf; vectors input HeA.movecs reorder 2 1 output HeB.movecs
end
task scf
et
vectors reactants HeA.movecs
vectors products HeB.movecs
end
task scf et
```
Here is what the output looks like for this example:
```
Electron Transfer Calculation
-----------------------------
MO vectors for reactants: HeA.movecs
MO vectors for products : HeB.movecs
Electronic energy of reactants H(RR) -5.2836825646
Electronic energy of products H(PP) -5.2836825646
Reactants/Products overlap S(RP) : -4.20D-04
Reactants/Products interaction energy:
-------------------------------------
One-electron contribution H1(RP) 0.0027017960
Beginning calculation of 2e contribution
Two-electron integral screening (tol2e) : 4.20D-11
Two-electron contribution H2(RP) -0.0004625156
Total interaction energy H(RP) 0.0022392804
Electron Transfer Coupling Energy |V(RP)| 0.0000220152
4.832 cm-1
0.000599 eV
0.014 kcal/mol
```
The overlap between the ET reactant and product states (SRP) is
small, so the magnitude of the coupling between the states is also
small. If the fragment guess or charged atomic density guess were not
used, the Mulliken spin population would be 0.5 on both He atoms, the
overlap between the ET reactant and product states would be 100% and an
infinite VRP would result.