5.6.4 Tuned RSH Functionals

Whereas the range-separation parameters for the functionals described in Section 5.6.2 are wholly empirical in nature and should not be adjusted, for the functionals described in Section 5.6.3 some adjustment was suggested, especially for TDDFT calculations and for any properties that require interpretation of orbital energies, such as HOMO/LUMO gaps. This adjustment can be performed in a non-empirical (albeit system-specific) way, ⁶⁰ Baer R., Livshits E., Salzner U.
Annu. Rev. Phys. Chem.
(2010), 61, pp. 85. Link by “tuning” the value of $\omega$ in order to satisfy the Koopmans-like ionization energy criterion

where $\text{IE}=E(N-1)-E(N)$ is the $\mathrm{\Delta }$SCF value of the ionization energy for the $N$-electron system of interest. The condition ${ϵ}_{\mathrm{HOMO}}=-\text{IE}$ is a theorem in exact DFT, ¹⁰⁶⁵ Perdew J. P., Levy M.
Phys. Rev. B
(1997), 56, pp. 16021. Link but this condition is often badly violated by approximate functionals. When an RSH functional is used, both sides of Eq. (5.18) are $\omega$-dependent and this parameter is adjusted until the condition in Eq. (5.18) is met, which requires a sequence of SCF calculations on both the neutral and ionized species, using different values of $\omega$. The value that is obtained has come to be called the “optimally tuned” value of $\omega$. Formally speaking, there is no guarantee that an approximate density functional can be made to satisfy Eq. (5.18) for any given molecule, thus the optimally-tuned value need not exist. In practice it is usually possible to find such a value, although it should be noted that the optimally-tuned value of $\omega$ depends on system size, and as a result this tuning procedure formally violates size-consistency. ⁶⁶⁷ Karolewski A., Kronik L., Kümmel S.
J. Chem. Phys.
(2013), 138, pp. 204115. Link

A few variations on the simple “IE tuning” criterion in Eq. (5.18) are possible. For proper description of charge-transfer states, Baer and co-workers ⁶⁰ Baer R., Livshits E., Salzner U.
Annu. Rev. Phys. Chem.
(2010), 61, pp. 85. Link suggest finding the value of $\omega$ that (to the extent possible) satisfies Eq. (5.18) for both the neutral donor molecule and (separately) for the anion of the acceptor species. Along similar lines, in an effort to set both the HOMO and LUMO energy levels such that the fundamental gap ($\text{IE}-\text{EA}$) is equal to the HOMO/LUMO gap, Kronik et al. ⁷²⁰ Kronik L. et al.
J. Chem. Theory Comput.
(2012), 8, pp. 1515. Link suggest minimizing the function

with respect to $\omega$, where $\text{EA}=E(N+1)-E(N)$. Minimization of $J(\omega )$ represents an attempt to satisfy the IE theorem of Eq. (5.18) for both the $N$-electron molecule and its ($N+1$)-electron anion, assuming that the latter is bound. Published benchmarks suggest that these system-specific approaches afford the most accurate values of IEs and TDDFT excitation energies. ⁸⁵¹ Livshits E., Baer R.
Phys. Chem. Chem. Phys.
(2007), 9, pp. 2932. Link ^{, 1195} Salzner U., Baer R.
J. Chem. Phys.
(2009), 131, pp. 231101. Link ^{, 60} Baer R., Livshits E., Salzner U.
Annu. Rev. Phys. Chem.
(2010), 61, pp. 85. Link ^{, 720} Kronik L. et al.
J. Chem. Theory Comput.
(2012), 8, pp. 1515. Link

A script that optimizes $\omega$, called OptOmegaIPEA.pl, is located in the $QC/bin/tools directory. The script scans $\omega$ over the range 0.1–0.8 bohr${}^{-1}$, corresponding to values of the $rem variable OMEGA in the range 100–800. See the script for the instructions how to modify the script to scan over a wider range. To execute the script, the user must create three inputs for an RSH single-point energy calculation, using the same geometry and basis set: one for a neutral molecule (N.in), one for its anion (M.in), and one for the molecule’s cation (P.in). The user should then run the command

OptOmegaIPEA.pl >& optomega

This command both generates the input files (N_*, P_*, M_*) and also runs Q-Chem on these input files, writing the optimization output into optomega. This script applies the IE condition to both the neutral molecule and its anion, minimizing $J(\omega )$ in Eq. (5.19). A similar script, OptOmegaIP.pl, uses Eq. (5.18) for the neutral molecule only.

Note:  1. If the system does not have positive EA, then the tuning should be done according to the IP condition only. The IP/EA script will yield an incorrect value of $\omega$ in such cases. 2. In order for the scripts to work, one must specify SCF_FINAL_PRINT = 1 in the inputs. The scripts look for specific regular expressions and will not work correctly without this keyword.

Although the tuning procedure was originally developed by Baer and co-workers using the BNL functional, ⁸⁵¹ Livshits E., Baer R.
Phys. Chem. Chem. Phys.
(2007), 9, pp. 2932. Link ^{, 1195} Salzner U., Baer R.
J. Chem. Phys.
(2009), 131, pp. 231101. Link ^{, 60} Baer R., Livshits E., Salzner U.
Annu. Rev. Phys. Chem.
(2010), 61, pp. 85. Link it can equally well be applied to any RSH functional, as for example LRC-$\omega$PBE (see, Ref.  1380 Uhlig F. et al.
J. Phys. Chem. A
(2014), 118, pp. 7507. Link ). The aforementioned scripts will work with these other RSH functionals as well.

Unfortunately, optimally-tuned values of “${\omega }_{\text{IE}}$”, obtained by satisfying the criterion in Eq. (5.18) exhibit a troubling dependence on system size, ⁷¹⁰ Körzdörfer T. et al.
J. Chem. Phys.
(2011), 135, pp. 204107. Link ^{, 1380} Uhlig F. et al.
J. Phys. Chem. A
(2014), 118, pp. 7507. Link ^{, 429} Garrett K. et al.
J. Chem. Theory Comput.
(2014), 10, pp. 3821. Link ^{, 1395} Vazquez X. A. S., Isborn C. M.
J. Chem. Phys.
(2015), 143, pp. 244105. Link ^{, 267} Coons M. P., You Z.-Q., Herbert J. M.
J. Am. Chem. Soc.
(2016), 138, pp. 10879. Link ^{, 1032} Oviedo M. B., Ilawe N. V., Wong B. M.
J. Chem. Theory Comput.
(2016), 12, pp. 3593. Link leading to a loss of size-extensivity. ⁶⁶⁷ Karolewski A., Kronik L., Kümmel S.
J. Chem. Phys.
(2013), 138, pp. 204115. Link For example, the optimally-tuned value for the cluster anion (H${}_2$O)${}^{-}_6$ is very different than the one tuned for (H${}_2$O)${}^{-}_{70}$, ¹³⁸⁰ Uhlig F. et al.
J. Phys. Chem. A
(2014), 118, pp. 7507. Link and the optimally-tuned value also varies with conjugation length for $\pi$-conjugated systems. ⁷¹⁰ Körzdörfer T. et al.
J. Chem. Phys.
(2011), 135, pp. 204107. Link An alternative to the IE-based criterion in Eq. (5.18) is the global density-dependent (GDD) tuning procedure, ⁹⁵⁶ Modrzejewski M. et al.
J. Phys. Chem. A
(2013), 117, pp. 11580. Link in which the optimal value

is related to the average of the distance $d_{\text{x}}$ between an electron in the outer regions of a molecule and the exchange hole in the region of localized valence orbitals. The quantity $C$ is an empirical parameter for a given LRC functional, which was determined for LRC-$\omega$PBE ($C=0.90$) and LRC-$\omega$PBEh ($C=0.75$), ⁹⁵⁶ Modrzejewski M. et al.
J. Phys. Chem. A
(2013), 117, pp. 11580. Link in order to ensure that ${\omega }_{\text{GDD}}\approx {\omega }_{\text{IE}}$ in small molecules. (A slightly different value, $C=0.885$, was determined for Q-Chem’s implementation of LRC-$\omega$PBE. ⁷⁶⁴ Lao K. U., Herbert J. M.
J. Chem. Theory Comput.
(2018), 14, pp. 2955. Link ) For TD-DFT calculations in small molecules, the choice of ${\omega }_{\text{GDD}}$ versus ${\omega }_{\text{IE}}$ makes essentially no difference, while the former can be considerably cheaper (and more convenient) as compared to searching for the value ${\omega }_{\text{IE}}$. ⁸⁸⁰ Mandal A., Herbert J. M.
J. Phys. Chem. Lett.
(2025), 16, pp. 2672. Link

Since LRC-${\omega }_{\text{GDD}}$PBE provides a better description of polarizabilities in polyacetylene as compared to ${\omega }_{\text{IE}}$, ⁵²⁶ Hapka M. et al.
J. Chem. Phys.
(2014), 141, pp. 134120. Link it is anticipated that using ${\omega }_{\text{GDD}}$ in place of ${\omega }_{\text{IE}}$ may afford more accurate molecular properties, especially in conjugated systems where IE tuning sometimes encounters problems related to vanishing HOMO/LUMO gaps. ⁴⁸⁰ Gray M., Herbert J. M.
J. Chem. Phys.
(2021), 155, pp. 034103. Link ^{, 880} Mandal A., Herbert J. M.
J. Phys. Chem. Lett.
(2025), 16, pp. 2672. Link GDD tuning of an RSH functional is involving by setting the $rem variable OMEGA_GDD = TRUE. The electron density is needed to compute ${\omega }_{\text{GDD}}$ in Eq. (5.20) and this is accomplished using the converged SCF density computed using the RSH functional with the value of $\omega$ given by the $rem variable OMEGA. The value of ${\omega }_{\text{GDD}}$ therefore depends, in principle, upon the value of OMEGA, although in practice it is not very sensitive to this value and results obtained in this way are similar to fully self-consistent values of ${\omega }_{\text{GDD}}$. ⁹⁵⁶ Modrzejewski M. et al.
J. Phys. Chem. A
(2013), 117, pp. 11580. Link

However the tuning is accomplished, these tuned functionals are generally thought to work by reducing self-interaction error in approximate DFT. A convenient way to quantify—or at least depict—this error is by plotting the DFT energy as a function of the (fractional) number of electrons, $N$, because $E(N)$ should in principle consist of a sequence of line segments with abrupt changes in slope (the so-called derivative discontinuity ²⁶¹ Cohen A. J., Mori-Sánchez P., Yang W.
Science
(2008), 321, pp. 792. Link ^{, 961} Mori-Sánchez P., Cohen A. J.
Phys. Chem. Chem. Phys.
(2014), 16, pp. 14378. Link ) at integer values of $N$, but in practice these $E(N)$ plots bow away from straight-line segments. ²⁶¹ Cohen A. J., Mori-Sánchez P., Yang W.
Science
(2008), 321, pp. 792. Link Examination of such plots has been suggested as a means to adjust the fraction of short-range exchange in an RSH functional, ⁵⁸ Autschbach J., Srebro M.
Acc. Chem. Res.
(2014), 47, pp. 2592. Link while the $\omega$ parameter is set by tuning.

Another, alternative to IE-based tuning is the effective (EFF) tuning procedure  ¹²⁵⁷ Singh A. et al.
J. Phys. Chem. Lett.
(2025), 16, pp. 8198. Link , in which the optimal value of the range-separation parameter is given by

where $a_1=1.91718$, $a_2=-0.02817$, and $a_3=0.14954$. The local Seitz radius is defined as $r_s={\left(\frac{3}{4\pi n}\right)}^{1/3}$, with $n=n_{\uparrow }+n_{\downarrow }$. The quantity $⟨r_s⟩$ denotes the volume-averaged Seitz radius.

While $⟨r_s⟩$ is straightforward to compute for bulk solids, special care is required for finite systems such as atoms and molecules, where $r_s$ diverges in the low-density tail. To address this issue, a weighted average of $r_s$ is employed:

Here, the weight function $w(𝐫)$ is constructed to emphasize regions where the electron density is significant (i.e., core and valence regions). For further details on the construction of the weight function and its derivation, we refer to Ref.  ¹²⁵⁷ Singh A. et al.
J. Phys. Chem. Lett.
(2025), 16, pp. 8198. Link .

The LRC-${\omega }_{\text{EFF}}$PBE functional has been shown to provide an improved description of charge-transfer excitations, HOMO-LUMO gaps, and excitonic properties.

In practice, EFF tuning of an RSH functional is enabled by setting the $rem variable OMEGA_EFF = TRUE. The evaluation of ${\omega }_{\text{EFF}}$ in Eq. (5.21) requires the electron density, which is obtained from a converged SCF calculation. When using an RSH functional, an initial value of $\omega$ must be specified via the $rem variable OMEGA. In contrast, for semilocal or global hybrid functionals (e.g., PBE or B3LYP), specifying OMEGA is not required.