13.5.2 Analysis of Charge-Transfer Based on Complementary Occupied/Virtual Pairs

In addition to quantifying the amount and energetics of intermolecular charge transfer, it is often useful to have a simple description of orbital interactions in intermolecular complexes. The polarized ALMOs obtained from the SCF MI procedure and used as a reference basis set in the decomposition analysis do not directly show which occupied-virtual orbital pairs are of most importance in forming intermolecular bonds. By performing rotations of the polarized ALMOs within a molecule, it is possible to find a “chemist’s basis set” that represents bonding between molecules in terms of just a few localized orbitals called complementary occupied-virtual pairs (COVPs). This orbital interaction model validates existing conceptual descriptions of intermolecular bonding. For example, in the modified ALMO basis, hydrogen bonding in water dimer is represented as an electron pair localized on an oxygen atom donating electrons to the O–H $\sigma$-antibonding orbital on the other molecule,⁴⁶⁵ and the description of synergic bonding in metal complexes agrees well with simple Dewar-Chatt-Duncanson model.^{181, 464, 592}

Set EDA_COVP to TRUE to perform the COVP analysis of the CT term in an EDA job. COVP analysis is currently implemented only for systems of two fragments. Set EDA_PRINT_COVP to TRUE to print out localized orbitals that form occupied-virtual pairs. In this case, MOs obtained in the end of the run (SCF MI orbitals, SCF MI(RA) orbitals, converged SCF orbitals) are replaced by the orbitals of COVPs. Each orbital is printed with the corresponding CT energy term in kJ/mol (instead of the energy eigenvalues in hartrees). These energy labels make it easy to find correspondence between an occupied orbital on one molecule and the virtual orbital on the other molecule. The examples below show how to print COVP orbitals. One way is to set $rem variable PRINT_ORBITALS, the other is to set IANLTY to 200 and use the $plots section in the Q-Chem input. In the first case the orbitals can be visualized using MOLDEN (set MOLDEN_FORMAT to TRUE), in the second case use VMD or a similar third party program capable of making 3D plots.