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, and, \(\beta\), 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