As pointed out in Ref. 291 and elsewhere, the description of charge-transfer excited states within density functional theory (or more precisely, time-dependent DFT, which is discussed in Section 7.3) requires full (100%) non-local HF exchange, at least in the limit of large donor–acceptor distance. Hybrid functionals such as B3LYP78, 1081 and PBE020 that are well-established and in widespread use, however, employ only 20% and 25% HF exchange, respectively. While these functionals provide excellent results for many ground-state properties, they cannot correctly describe the distance dependence of charge-transfer excitation energies, which are enormously underestimated by most common density functionals. This is a serious problem in any case, but it is a catastrophic problem in large molecules and in non-covalent clusters, where TDDFT often predicts a near-continuum of spurious, low-lying charge transfer states.621, 623 The problems with TDDFT’s description of charge transfer are not limited to large donor–acceptor distances, but have been observed at 2 Å separation, in systems as small as uracil–(HO).621 Rydberg excitation energies also tend to be substantially underestimated by standard TDDFT.
One possible avenue by which to correct such problems is to parameterize functionals that contain 100% HF exchange, though few such functionals exist to date. An alternative option is to attempt to preserve the form of common GGAs and hybrid functionals at short range (i.e., keep the 25% HF exchange in PBE0) while incorporating 100% HF exchange at long range, which provides a rigorously correct description of the long-range distance dependence of charge-transfer excitation energies, but aims to avoid contaminating short-range exchange-correlation effects with additional HF exchange. The separation is accomplished using the range-separation ansatz that was introduced in Section 5.3. In particular, functionals that use 100% HF exchange at long range ( in Eq. (5.13)) are known as “long-range-corrected” (LRC) functionals. An LRC version of PBE0 would, for example, have .
To fully specify an LRC functional, one must choose a value for the range separation parameter in Eq. (5.12). In the limit , the LRC functional in Eq. (5.13) reduces to a non-RSH functional where there is no “SR” or “LR”, because all exchange and correlation energies are evaluated using the full Coulomb operator, . Meanwhile the limit corresponds to a new functional, . Full HF exchange is inappropriate for use with most contemporary GGA correlation functionals, so the latter limit is expected to perform quite poorly. Values of bohr are likely not worth considering, according to benchmark tests.628, 973
Evaluation of the short- and long-range HF exchange energies is straightforward,24 so the crux of any RSH functional is the form of the short-range GGA exchange functional, and several such functionals are available in Q-Chem. These include short-range variants of the B88 and PBE exchange described by Hirao and co-workers,500, 1058 called B88 and PBE in Q-Chem,966 and an alternative formulation of short-range PBE exchange proposed by Scuseria and co-workers,450 which is known as PBE. These functionals are available in Q-Chem thanks to the efforts of the Herbert group.973, 974 By way of notation, the terms “PBE”, “PBE”, etc., refer only to the short-range exchange functional, in Eq. (5.13). These functionals could be used in “screened exchange” mode, as described in Section 5.3, as for example in the HSE03 functional,464 therefore the designation “LRC-PBE”, for example, should only be used when the short-range exchange functional PBE is combined with 100% Hartree-Fock exchange in the long range.
In general, LRC-DFT functionals have been shown to remove the near-continuum of spurious charge-transfer excited states that appear in large-scale TDDFT calculations.628 However, certain results depend sensitively upon the value of the range-separation parameter ,628, 973, 974, 623, 1141 especially in TDDFT calculations (Section 7.3) and therefore the results of LRC-DFT calculations must therefore be interpreted with caution, and probably for a range of values. This can be accomplished by requesting a functional that contains some short-range GGA exchange functional (PBE or PBE, in the examples mentioned above), in combination with setting the $rem variable LRC_DFT = TRUE, which requests the addition of 100% Hartree-Fock exchange in the long-range. Basic job-control variables and an example can be found below. The value of the range-separation parameter is then controlled by the variable OMEGA, as shown in the examples below.
$comment The value of omega is 0.47 by default but can be overwritten by specifying OMEGA. $end $molecule -1 2 O 1.347338 -0.017773 -0.071860 H 1.824285 0.813088 0.117645 H 1.805176 -0.695567 0.461913 O -1.523051 -0.002159 -0.090765 H -0.544777 -0.024370 -0.165445 H -1.682218 0.174228 0.849364 $end $rem EXCHANGE LRC-BOP BASIS 6-311(1+,2+)G* XC_GRID 2 LRC_DFT TRUE OMEGA 300 ! = 0.300 bohr**(-1) $end
Rohrdanz et al.974 published a thorough benchmark study of both ground- and excited-state properties using the LRC-PBEh functional, in which the “h” indicates a short-range hybrid (i.e., the presence of some short-range HF exchange). Empirically-optimized parameters of (see Eq. (5.13)) and bohr were obtained,974 and these parameters are taken as the defaults for LRC-PBEh. Caution is warranted, however, especially in TDDFT calculations for large systems, as excitation energies for states that exhibit charge-transfer character can be rather sensitive to the precise value of .623, 974 In such cases (and maybe in general), the “tuning” procedure described in Section 5.6.4 is recommended.
$comment This example uses the "optimal" parameter set discussed above. It can also be run by setting METHOD = LRC-wPBEh. $end $molecule 0 1 C 0.670604 0.000000 0.000000 C -0.670604 0.000000 0.000000 H 1.249222 0.929447 0.000000 H 1.249222 -0.929447 0.000000 H -1.249222 0.929447 0.000000 H -1.249222 -0.929447 0.000000 C 0.669726 0.000000 5.000000 C -0.669726 0.000000 5.000000 F 1.401152 1.122634 5.000000 F 1.401152 -1.122634 5.000000 F -1.401152 -1.122634 5.000000 F -1.401152 1.122634 5.000000 $end $rem EXCHANGE GEN BASIS 6-31+G* LRC_DFT TRUE OMEGA 200 ! = 0.2 a.u. CIS_N_ROOTS 4 CIS_TRIPLETS FALSE $end $xc_functional C PBE 1.00 X wPBE 0.80 X HF 0.20 $end
By adding 100% Hartree-Fock exchange to the asymptotic Coulomb operator, LRC functionals guarantee that an electron and hole experience an asymptotic interaction potential . This is correct for a molecule in the gas phase, but to simulate a material one might desire an asymptotic behavior of , where is the (static) dielectric constant of the material. In conjunction with “optimal tuning” of the range-separation parameter, as described in Section 5.6.4, such functionals have been shown to afford accurate fundamental gaps for organic photovoltaic materials,593 and are naturally combined with polarizable continuum models (Section 11.2.3) that employ the same dielectric constant.107 These have come to be called screened RSH (sRSH) functionals.593 An XC function of this type can be expressed generically as32
which should be compared to Eq. (5.13) that provides the generic form for an RSH functional. Although the RSH formalism allows for an arbitrary coefficient for the long-range Hartree-Fock exchange term, as in Eq. (5.13), this implies that the asymptotic electron–hole interaction has the form rather than .291 As such, LRC functionals are a particular class of RSH functionals where , ensuring proper asymptotic behavior in vacuum. Along the same lines, sRSH functionals set to ensure proper asymptotic behavior in a dielectric material. Using Eq. (5.17), users may construct sRSH functionals by means of a $xc_functional input section.