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, ε_HOMO = -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.