The Trip to the Density Functional Theory Zoo Continues: Making a Case for Time-Dependent Double Hybrids for Excited-State Problems

Abstract

This account is written for general users of time-dependent density functional theory (TD-DFT) methods as well as chemists who are unfamiliar with the field. It includes a brief overview of conventional TD-DFT approaches and recommendations for applications to organic molecules based on our own experience. The main emphasis of this work, however, lies in providing the first in-depth review of time-dependent double-hybrid density functionals. They were first established in 2007 with very promising follow-up studies in the subsequent four years before developments or applications became scarce. The topic has regained more interest since 2017, and this account reviews those latest developments led by our group. These developments have shown unprecedented robustness for a variety of different types of electronic excitations when compared to more conventional TD-DFT methods. In particular, time-dependent double hybrids do not suffer from artificial ghost states and are able to reproduce exciton-coupled absorption spectra. Our latest methods include range separation and belong to the currently best TD-DFT methods for singlet-singlet excitations in organic molecules. While there is still room for improvement and further development in this space, we hope that this account encourages users to adjust their computational protocols to such new methods to provide more real-life testing and scenarios.

Organic molecules with inverted gaps between first excited singlet and triplet states and appreciable fluorescence rates

“…It accounts for double excitations perturbatively based on a CIS(D) 155,156 calculation, and it was shown to be one of the most reliable TD-DFA methods currently available. 122 Generally, it reproduces trends in both singlet-triplet gaps and oscillator strengths faithfully. Occasional outliers of singlet-triplet gaps lead to missing molecules that would be inverted, but not to misidentifying ordinary structures as INVEST molecules (Figure 3a).…”

“…22,116 Therefore, refined models accounting for double excitations are required to describe the low-energy excited electronic states of INVEST emitters appropriately. TD-DFAs need to be double-hybrid or higher in their rung 41,122 or have to be used as spin-flip variant with a well-behaved triplet state as reference. 123 Accordingly, a four-state model has been proposed as minimal model to describe the low-energy excited states of INVEST molecules.…”

“…Hence, they also support the minimal four-state model developed recently describing singlet-triplet gaps of organic emitters with the correct physics. 116 Based on comprehensive benchmarking against ADC(2), [44][45][46][47][48][49][50] EOM-CCSD [51][52][53][54][55] and FNO-EOM-CCSD [51][52][53][54][55][56][57][58][59] calculations as references, we established a robust computational protocol using ωB2PLP, 41 a doublehybrid TD-DFA, [68][69][70][71]122 as our computational workhorse (Figure 3). It accounts for double excitations perturbatively based on a CIS(D) 155,156 calculation, and it was shown to be one of the most reliable TD-DFA methods currently available.…”

A Simple Range-Separated Double-Hybrid Density Functional Theory for Excited States

“… 77 − 80 The connection between the fraction of the exact exchange in the RS XC energy and the optimal range-separation parameter for the nonempirical functionals was presented by Adamo et al 78 Later, it was demonstrated that these parameters are in line with the results obtained by relying on the so-called optimally tuned procedure. 79 The only excited-state RS-DH approach proposed 81 and carefully benchmarked 82 , 83 recently by Goerigk and co-workers also originated from this scheme.…”

RACI and AAS Awards 2018–2020

“…Lars Goerigk and co-author Marcos Casanova-Paéz (University of Melbourne) start out with an account, 'The Trip to the Density Functional Theory Zoo Continues: Making a Case for Time-Dependent Double Hybrids for Excited-State Problems', [1] which is written for the general users of timedependent (TD) density functional theory (DFT). It is written in a way that also makes it useful for scientists who have little prior knowledge in the area.…”