Charge-Transfer Excitations within Density Functional Theory: How Accurate Are the Most Recommended Approaches?

The performance of
the most recent density functionals is assessed
for charge-transfer (CT) excitations using comprehensive intra- and
intermolecular CT benchmark sets with high-quality reference values.
For this comparison, the state-of-the-art range-separated (RS) and
long-range-corrected (LC) double hybrid (DH) approaches are selected,
and global DH and LC hybrid functionals are also inspected. The correct
long-range behavior of the exchange–correlation (XC) energy
is extensively studied, and various CT descriptors are compared as
well. Our results show that the most robust performance is attained
by RS-PBE-P86/SOS-ADC(2), as it is suitable to describe both types
of CT excitations with outstanding accuracy. Furthermore, concerning
the intramolecular transitions, unexpectedly excellent results are
obtained for most of the global DHs, but their limitations are also
demonstrated for bimolecular complexes. Despite the outstanding performance
of the LC-DH methods for common intramolecular excitations, serious
deficiencies are pointed out for intermolecular CT transitions, and
the wrong long-range behavior of the XC energy is revealed. The application
of LC hybrids to such transitions is not recommended in any respect.