Excited State Properties of Organic Semiconductors: Breakdown of the Tamm-Dancoff Approximation

Exciton-plasmon states in nanoscale materials: breakdown of the Tamm-Dancoff approximation

Within the Tamm-Dancoff approximation ab initio approaches describe excitons as packets of electron-hole pairs propagating only forward in time. However, we show that in nanoscale materials excitons and plasmons hybridize, creating exciton--plasmon states where the electron-hole pairs oscillate back and forth in time. Then, as exemplified by the trans-azobenzene molecule and carbon nanotubes, the Tamm-Dancoff approximation yields errors as large as the accuracy claimed in ab initio calculations. Instead, we propose a general and efficient approach that avoids the Tamm--Dancoff approximation, a…

Excitons in solids from time-dependent density-functional theory: Assessing the Tamm-Dancoff approximation

Excitonic effects in solids can be calculated using the Bethe-Salpeter equation (BSE) or the Casida equation of time-dependent density-functional theory (TDDFT). In both methods, the Tamm-Dancoff approximation (TDA), which decouples excitations and de-excitations, is widely used to reduce computational cost. Here, we study the effect of the TDA on exciton binding energies of solids obtained from the Casida equation using long-range corrected (LRC) exchange-correlation kernels. We find that the TDA underestimates TDDFT-LRC exciton binding energies of semiconductors slightly, but those of insula…

推进TADF发射体的激发态模拟：用于高通量筛选与设计的扩展紧束缚框架

Advancing excited-state simulations for TADF emitters: An eXtended Tight-Binding framework for high-throughput screening and design

该研究提出一种结合扩展紧束缚、简化Tamm-Dancoff近似和简化含时密度泛函理论的计算框架，用于高效预测TADF发射体的激发态性质，实现超过99%的计算成本降低，并揭示扭转角与溶剂诱导红移的相关性，为高通量筛选和材料设计提供新途径。

Solving the Bethe-Salpeter Equation on a Subspace: Approximations and Consequences for Low-dimensional Materials

It is well known that the ambient environment can dramatically renormalize the quasiparticle gap and exciton binding energies in low-dimensional materials, but the effect of the environment on the energy splitting of the spin-singlet and spin-triplet exciton states is less understood. A prominent effect is the renormalization of the exciton binding energy and optical strength (and hence the optical spectrum) through additional screening of the direct Coulomb term describing the attractive electron-hole interaction in the kernel of the Bethe-Salpeter equation (BSE). The repulsive exchange inter…

Three-particle states and brightening of intervalley excitons in a doped MoS$_2$ monolayer

Optical spectra of two-dimensional transition-metal dichalcogenides (TMDC) are influenced by complex multi-particle excitonic states. Their theoretical analysis requires solving the many-body problem, which in most cases, is prohibitively complicated. In this work, we calculate the optical spectra by exact diagonalization of the three-particle Hamiltonian within the Tamm-Dancoff approximation where the doping effects are accounted for via the Pauli blocking mechanism, modelled by a discretized mesh in the momentum space. The single-particle basis is extracted from the {\it ab initio} calculati…