Limitations of Global Hybrids in Predicting the Geometries and Torsional Energy Barriers of Dimeric Systems and the Role of Hartree Fock and DFT Exchange

Abstract

Prediction of accurate geometries is a prerequisite for accurate prediction of molecular properties. Impact of Hartree Fock (HF) exchange (a0) on geometry in the framework of DFT is investigated by monitoring dihedral angles, bond length alternations, and torsional energy barriers of 10 dimeric systems against CCSD (ADZ/ATZ) benchmarks. A strong correlation is observed between the fraction of HF exchange, equilibrium dihedral angles, and the potential energy barriers in global hybrids. Full HF exchange is critical to accurately predict the nonplanarity. Lower fractions of (a0)/larger DFT exchange (1‐a0) results in overestimation of torsional energy barriers at 900 and underestimation at 00. Large contributions of (1‐a0) in global hybrid functionals tend to overestimate torsional energy barriers (900) and are biased toward planar geometries. However, inclusion of larger fractions of (a0)/lower (1‐a0) also overestimate the torsional energy barriers in syn‐conformations due to the localization errors associated with HF exchange in global hybrids. Hence, irrespective of the fraction of HF/DFT exchange incorporated, global hybrids fail to accurately predict torsional energy barriers at 00 and 900 simultaneously. Long‐range corrected (LC) functionals, which employ full HF exchange at longer regions, outperform global hybrid functionals in predicting geometries and torsional energy barriers of the dimeric molecules. The distance dependence of (a0) thus provides a balanced fraction of HF exchange as the dihedral torsion varies. Impact of range separation parameter on geometries is marginal in altering the planarity/nonplanarity. However, range separation parameter within 0.20–0.40 bohr−1 predicts more reliable torsional energies and geometries. © 2019 Wiley Periodicals, Inc.

Accurate Prediction of Vertical Emission from Excited-State Tuning of Range-Separated Density Functional Theory

“…Tuning approach in LC hybrid functionals seems to be a promising approach in this direction. , The main drawback of the conventional GH functionals (and LC-hybrids) which are widely used is that they incorporate a constant fraction of HF exchange [or a constant range-separation parameter (RSP)], whereas the HF exchange incorporated is size-dependent (varies with the size and geometry of the molecular system) . By tuning the RSP in the LC-hybrids, the optimum HF exchange required is incorporated. ,, …”

“…), it is still not clear if this approach simultaneously predicts accurate vertical emission (VE). Theoretical methods such as DFT (LC functionals) generally provide ground-state (GS) geometries closer to experiments. , In the tuning approach, the GS geometry is considered and the HF exchange is optimized with respect to the fundamental gap (see next section for more details), hence resulting in the accurate prediction of VA. ,,, However, upon excitation, the GS molecular geometry alters and the delocalization patterns and energy gaps change. Therefore, the HF exchange which is optimized based on the GS geometry may/may not ensure the accurate prediction of VE.…”

Novel Donor–Electret–Acceptor Framework for Higher Charge Transfer and Distance of Charge Transfer through Dipole Engineering

“…“ω” reduces from 0.22 to 0.17 bohr –1 for CP series and from 0.24to 0.19 bohr –1 with the thiophene series. The decrease in “ω” suggests the incorporation of a larger fraction of DFT exchange, which makes the molecular system to exhibit more delocalization , with an increasing number of bridge/electret units ( n = 2–9). Hence, we notice q CT / D CT to be more sensitive with varying chain lengths for CP and thiophene series.…”

“…All the geometry optimizations (DFT) and TD-DFT calculations are carried out employing LR corrected LC-ωHPBE with the 6-311++G­(d) basis set, which provides accurate molecular geometries compared to the standard global hybrids . Ground-state (GS) geometry optimizations are performed with the default range separation parameter (0.40 bohr –1 ) of LC-ωHPBE since the range separation parameter between 0.20 and 0.40 bohr –1 is adequate enough to obtain accurate molecular geometries . A tuning approach is employed to derive the optimum range separation parameter for each molecular system investigated. , The PCM solvation model is adopted to mimic the solvent (toluene) environment.…”

“…22 Ground-state (GS) geometry optimizations are performed with the default range separation parameter (0.40 bohr −1 ) of LC-ωHPBE since the range separation parameter between 0.20 and 0.40 bohr −1 is adequate enough to obtain accurate molecular geometries. 22 A tuning approach is employed to derive the optimum range separation parameter for each molecular system investigated. 23,24 The PCM 25 solvation model is adopted to mimic the solvent (toluene) environment.…”