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 -antibonding orbital on the
other molecule,
644
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pp. 851.
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and the description of synergic bonding
in metal complexes agrees well with simple Dewar-Chatt-Duncanson
model.
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(2007),
104,
pp. 6963.
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,
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J. Chem. Phys.
(2008),
128,
pp. 184112.
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,
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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 Visual Molecular Dynamics (VMD)W. Humphrey, A. Dalke, and K. Schulten (1996), 15 or a similar third party program capable of making 3D plots.
Example 12.10 COVP analysis of the CT term. The COVP orbitals are printed in the Q-Chem and MOLDEN formats.
$molecule 0 1 -- 0 1 O -1.521720 0.129941 0.000000 H -1.924536 -0.737533 0.000000 H -0.571766 -0.039961 0.000000 -- 0 1 O 1.362840 -0.099704 0.000000 H 1.727645 0.357101 -0.759281 H 1.727645 0.357101 0.759281 $end $rem JOBTYPE EDA EDA2 FALSE BASIS 6-31G PURECART 1112 METHOD B3LYP FRGM_METHOD GIA FRGM_LPCORR RS_EXACT_SCF EDA_COVP TRUE EDA_PRINT_COVP TRUE PRINT_ORBITALS 16 MOLDEN_FORMAT TRUE $end
Example 12.11 COVP analysis of the CT term. Note that it is not necessary to run a full EDA job. It is suffice to set FRGM_LPCORR to RS or ARS and EDA_COVP = TRUE to perform the COVP analysis. The orbitals of the most significant occupied-virtual pair are printed into an ASCII file called plot.mo which can be converted into a cube file and visualized in VMD.
$molecule 0 1 -- 0 1 O -1.521720 0.129941 0.000000 H -1.924536 -0.737533 0.000000 H -0.571766 -0.039961 0.000000 -- 0 1 O 1.362840 -0.099704 0.000000 H 1.727645 0.357101 -0.759281 H 1.727645 0.357101 0.759281 $end $rem BASIS 6-31G PURECART 1112 METHOD B3LYP FRGM_METHOD GIA FRGM_LPCORR RS IANLTY 200 EDA_COVP TRUE EDA_PRINT_COVP TRUE $end $plots MOs 80 -4.0 4.0 60 -3.0 3.0 60 -3.0 3.0 2 0 0 0 6 11 $end