Electronic Couplings
Once electronic excited states of the fragments are selected, the
electronic couplings are computed. The model is based on point-dipole
approximation with a linear electrostatic screening factor (s ):
\(V_{\text{ij}}=\text{s\ V}_{\text{ij}}^{0}\) (7)
Alternatively, exponentially attenuated transition dipole moments can be
used in PyFREC:2, 3
\(V_{\text{ij}}=V_{\text{ij}}^{0}\left(\text{Aexp}\left(-\text{βR}\right)+\ s_{0}\right)\)(8)
where \(V_{\text{ij}}^{0}\) is the point-dipole electronic coupling, andA , \(\beta\), and \(s_{0}\) are parameters provided in the
input.24, 25 As the anisotropy of the protein
environment the screening may depend on the orientation of the
fragments38 the proposed approximation has to be used
with caution. The Förster coupling16, 17 can be split
into distance- and orientation-dependent parts:1-3
\({V_{\text{ij}}^{0}}=\frac{1}{R^{3}}\left|\mathbf{\mu}_{\mathbf{i}}\right|\left|\mathbf{\mu}_{\mathbf{j}}\right|K_{\text{ij}}\)(9)
where R is the distance between centroids of fragments,\(\left|\mu_{i}\right|,\) \(\left|\mu_{j}\right|\ \)are magnitudes
of transition dipole moments, and \(-1{\leq K}_{\text{ij}}/2\leq 1\)is the orientation factor which depends only on mutual orientations of
transition dipole moments. The centroids of fragments are defined as
centroids of the electric charge of the ground state electronic density
(origin in the standard orientation of Gaussian
software).15