Simplified Tuning of Long-Range Corrected Time-Dependent Density Functional Theory

Range-separated hybrid functionals have dramatically
improved the
description of charge-transfer excitations in time-dependent density
functional theory (TD-DFT), especially when the range-separation parameter
is adjusted in order to satisfy the ionization energy (IE) criterion,
εHOMO = −IE. However, this “optimal
tuning” procedure is molecule-specific, inconvenient, expensive
for large systems, and problematic in extended or periodic systems.
Here, we consider an alternative procedure known as global density-dependent
(GDD) tuning, which sets the range-separation parameter in an automated
way based on properties of the exchange hole. In small molecules,
we find that long-range corrected functionals with either IE or GDD
tuning afford remarkably similar TD-DFT excitation energies, for both
valence and charge-transfer excitations. However, GDD tuning is more
efficient and is well-behaved even for large systems. It provides
a black-box solution to the optimal-tuning problem that can replace
IE tuning for many applications of TD-DFT.