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1. Dispersionless orbital excitations in (Li,Fe)OHFeSe superconductors

   https://doi.org/10.1038/s41535-022-00492-0  

   Xiao, Qian; Zhang, Wenliang; Asmara, Teguh Citra; Li, Dong; Li, Qizhi; Zhang, Shilong; Tseng, Yi; Dong, Xiaoli; Wang, Yao; Chen, Cheng-Chien; et al (December 2022, npj Quantum Materials)

   Abstract The superconducting critical temperature T c of intercalated iron-selenide superconductor (Li,Fe)OHFeSe (FeSe11111) can be increased to 42 from 8 K of bulk FeSe. It shows remarkably similar electronic properties as the high- T c monolayer FeSe and provides a bulk counterpart to investigate the origin of enhanced superconductivity. Unraveling the nature of excitations is crucial for understanding the pairing mechanism in high- T c iron selenides. Here we use resonant inelastic x-ray scattering (RIXS) to investigate the excitations in FeSe11111. Our high-quality data exhibit several Raman-like excitations, which are dispersionless and isotropic in momentum transfer in both superconducting 28 K and 42 K samples. Using atomic multiplet calculations, we assign the low-energy ~0.3 and 0.7 eV Raman peaks as local e g  −  e g and e g  −  t 2 g orbital excitations. The intensity of these two features decreases with increasing temperature, suggesting a dominating contribution of the orbital fluctuations. Our results highlight the importance of the orbital degree of freedom for high- T c iron selenides. 

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2. Combining localized orbital scaling correction and Bethe–Salpeter equation for accurate excitation energies

   https://doi.org/10.1063/5.0087498  

   Li, Jiachen; Jin, Ye; Su, Neil Qiang; Yang, Weitao (April 2022, The Journal of Chemical Physics)

   We applied localized orbital scaling correction (LOSC) in Bethe–Salpeter equation (BSE) to predict accurate excitation energies for molecules. LOSC systematically eliminates the delocalization error in the density functional approximation and is capable of approximating quasiparticle (QP) energies with accuracy similar to or better than GW Green’s function approach and with much less computational cost. The QP energies from LOSC, instead of commonly used G 0 W 0 and ev GW, are directly used in BSE. We show that the BSE/LOSC approach greatly outperforms the commonly used BSE/ G 0 W 0 approach for predicting excitations with different characters. For the calculations of Truhlar–Gagliardi test set containing valence, charge transfer, and Rydberg excitations, BSE/LOSC with the Tamm–Dancoff approximation provides a comparable accuracy to time-dependent density functional theory (TDDFT) and BSE/ev GW. For the calculations of Stein CT test set and Rydberg excitations of atoms, BSE/LOSC considerably outperforms both BSE/ G 0 W 0 and TDDFT approaches with a reduced starting point dependence. BSE/LOSC is, thus, a promising and efficient approach to calculate excitation energies for molecular systems. 

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3. Linear-scaling quadruple excitations in local pair natural orbital coupled-cluster theory

   https://doi.org/10.1063/5.0257672  

   Jiang, Andy; Schaefer, Henry F; Turney, Justin M (April 2025, The Journal of Chemical Physics)

   We present a fast, asymptotically linear-scaling implementation of the perturbative quadruples energy correction in coupled-cluster theory using local natural orbitals. Our work follows the domain-based local pair natural orbital (DLPNO) approach previously applied to lower levels of excitations in coupled-cluster theory. Our DLPNO-CCSDT(Q) algorithm uses converged doubles and triples amplitudes from a preceding DLPNO-CCSDT computation to compute the quadruples amplitude and energy in the quadruples natural orbital (QNO) basis. We demonstrate the compactness of the QNO space, showing that more than 95% of the (Q) correction can be recovered using relatively loose natural orbital cutoffs, compared to the tighter cutoffs used in pair and triples natural orbitals at lower levels of coupled-cluster theory. We also highlight the accuracy of our algorithm in the computation of relative energies, which yields deviations of sub-kJ mol−1 in relative energy compared to the canonical CCSDT(Q). Timings are conducted on a series of growing linear alkanes (up to 10 carbons and 608 basis functions) and water clusters (up to 49 water molecules and 2842 basis functions) to establish the asymptotic linear-scaling of our DLPNO-(Q) algorithm. 

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4. Triple electron–electron–proton excitations and second-order approximations in nuclear–electronic orbital coupled cluster methods

   https://doi.org/10.1063/5.0106173  

   Pavošević, Fabijan; Hammes-Schiffer, Sharon (August 2022, The Journal of Chemical Physics)

   The accurate description of nuclear quantum effects, such as zero-point energy, is important for modeling a wide range of chemical and biological processes. Within the nuclear–electronic orbital (NEO) approach, such effects are incorporated in a computationally efficient way by treating electrons and select nuclei, typically protons, quantum mechanically with molecular orbital techniques. Herein, we implement and test a NEO coupled cluster method that explicitly includes the triple electron–electron–proton excitations, where two electrons and one proton are excited simultaneously, using automatic differentiation. Our calculations show that this NEO-CCSDT eep method provides highly accurate proton densities and proton affinities, outperforming any previously studied NEO method. These examples highlight the importance of the triple electron–electron–proton excitations for an accurate description of nuclear quantum effects. Additionally, we also implement and test the second-order approximate coupled cluster with singles and doubles (NEO-CC2) method as well as its scaled-opposite-spin (SOS) versions. The NEO-SOS′-CC2 method, which scales the electron–proton correlation energy as well as the opposite-spin and same-spin components of the electron–electron correlation energy, achieves nearly the same accuracy as the NEO-CCSDT eep method for the properties studied. Because of its low computational cost, this method will enable a wide range of chemical and photochemical applications for large molecular systems. This work sets the stage for a variety of developments and applications within the NEO framework. 

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5. Emergent flat band and topological Kondo semimetal driven by orbital-selective correlations

   https://doi.org/10.1038/s41467-024-49306-w  

   Chen, Lei; Xie, Fang; Sur, Shouvik; Hu, Haoyu; Paschen, Silke; Cano, Jennifer; Si, Qimiao (December 2024, Nature Communications)

   Flat electronic bands are expected to show proportionally enhanced electron correlations, which may generate a plethora of novel quantum phases and unusual low-energy excitations. They are increasingly being pursued in d-electron-based systems with crystalline lattices that feature destructive electronic interference, where they are often topological. Such flat bands, though, are generically located far away from the Fermi energy, which limits their capacity to partake in the low-energy physics. Here we show that electron correlations produce emergent flat bands that are pinned to the Fermi energy. We demonstrate this effect within a Hubbard model, in the regime described by Wannier orbitals where an effective Kondo description arises through orbital-selective Mott correlations. Moreover, the correlation effect cooperates with symmetry constraints to produce a topological Kondo semimetal. Our results motivate a novel design principle for Weyl Kondo semimetals in a new setting, viz. d-electron-based materials on suitable crystal lattices, and uncover interconnections among seemingly disparate systems that may inspire fresh understandings and realizations of correlated topological effects in quantum materials and beyond. 

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