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1. Correction: Dyson orbitals within the fc-CVS-EOM-CCSD framework: theory and application to X-ray photoelectron spectroscopy of ground and excited states

   https://doi.org/10.1039/d0cp90012e  

   Vidal, Marta L.; Krylov, Anna I.; Coriani, Sonia (February 2020, Physical Chemistry Chemical Physics)

   Correction for ‘Dyson Orbitals within the fc-CVS-EOM-CCSD framework: theory and application to X-ray photoelectron spectroscopy of ground and excited states’ by M. L. Vidal et al. , Phys. Chem. Chem. Phys. , 2020, DOI: 10.1039/C9CP03695D 

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2. How to stay out of trouble in RIXS calculations within equation-of-motion coupled-cluster damped response theory? Safe hitchhiking in the excitation manifold by means of core–valence separation

   https://doi.org/10.1039/c9cp03688a  

   Nanda, Kaushik D.; Vidal, Marta L.; Faber, Rasmus; Coriani, Sonia; Krylov, Anna I. (February 2020, Physical Chemistry Chemical Physics)

   We present a novel approach for computing resonant inelastic X-ray scattering (RIXS) cross sections within the equation-of-motion coupled-cluster (EOM-CC) framework. The approach is based on recasting the sum-over-states expressions for RIXS moments into closed-form expressions by using damped response theory. Damped response formalism allows one to circumvent problems of divergent behavior of response equations in the resonant regime. However, the convergence of response equations in the X-ray frequency range is often erratic due to the electronically metastable ( i.e. , resonant) nature of the virtual core-excited states embedded in the valence ionization continuum. We circumvent this problematic behavior by extending the core–valence separation (CVS) scheme, which decouples the valence-excited and core-excited configurations of the excitation manifold, into the response domain. The accuracy of the CVS-enabled damped response theory, implemented within the EOM-EE-CCSD (EOM-CC for excitation energies with single and double excitations) framework, is assessed by comparison against standard damped EOM-EE-CCSD response calculations. The capabilities of the new approach are illustrated by calculations of RIXS cross sections for benzene and benzene radical cation. 

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3. Molecular-Orbital Framework of Two-Electron Processes: Application to Auger and Intermolecular Coulomb Decay.

   https://doi.org/10.26434/chemrxiv-2023-4l9cz  

   Karippara_Jayadev, Nayanthara; Skomorowski, Wojciech; Krylov, Anna I (July 2023, J. Phys. Chem. Lett.)

   States with core vacancies, which are commonly created by absorption of X-ray photons, can decay by a two-electron process in which one electron fills the core hole and the second one is ejected. These processes accompany many X-ray spectroscopies. Depending on the nature of the initial core-hole state and the decay valence-hole states, these processes are called Auger decay, intermolecular Coulomb decay, or electron-transfer-mediated decay. To connect many-body wavefunctions of the initial and final states with molecular orbital picture of the decay, we introduce a concept of natural Auger orbitals (NAOs). NAOs are obtained by two-step singular value decomposition of the two-body Dyson orbitals, reduced quantities that enter the expression of the decay rate in the Feshbach--Fano treatment. NAOs afford chemical insight and interpretation of the high-level ab intio calculations of Auger decay and related two-electron relaxation processes. 

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4. Structures, energies and vibrational frequencies of the X and A states of haloacetylene cations, HCCX+ (X = F, Cl, Br, I)

   https://doi.org/10.1016/j.ijms.2024.117313  

   Durden, Andrew S; Caricato, Marco; Schlegel, H Bernhard (November 2024, International Journal of Mass Spectrometry)

   Modeling charge migration resulting from the coherent superposition of cation ground and excited states requires information about the potential energy surfaces of the relevant cation states. Since these states are often of the same electronic symmetry as the ground state of the cation, conventional single reference methods such as coupled cluster cannot be used for the excited states. The EOMCCSD-IP (equation of motion coupled cluster with single and double excitations and ionization) is a convenient and reliable “black-box” method that can be used for the ground and excited states of cations, yielding results of CCSD (coupled cluster with singles and double excitation) quality. Charge migration in haloacetylene cations arises from the superposition of the X and A states of HCCX+ (X = F, Cl, Br and I). The geometries, ionization potentials and vibrational frequencies have been calculated by CCSD/cc-pVTZ for neutral HCCX and the X state of HCCX+ and by EOM CCSD-IP/cc-pVTZ for the X and A states of HCCX+. The results agree very well with each other and with experiment. The very good agreement between CCSD and EOMCCSD-IP for the X states demonstrates that EOMCCSD-IP is a suitable method for calculating the structure and properties of ground and excited states for the HCCX cations. 

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5. Performance of periodic EOM-CCSD for bandgaps of inorganic semiconductors and insulators

   https://doi.org/10.1063/5.0187856  

   Vo, Ethan A; Wang, Xiao; Berkelbach, Timothy C (January 2024, The Journal of Chemical Physics)

   We calculate bandgaps of 12 inorganic semiconductors and insulators composed of atoms from the first three rows of the Periodic Table using periodic equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD). Our calculations are performed with atom-centered triple-zeta basis sets and up to 64 k-points in the Brillouin zone. We analyze the convergence behavior with respect to the number of orbitals and number of k-points sampled using composite corrections and extrapolations to produce our final values. When accounting for electron–phonon corrections to experimental bandgaps, we find that EOM-CCSD has a mean signed error of −0.12 eV and a mean absolute error of 0.42 eV; the largest outliers are C (error of −0.93 eV), BP (−1.00 eV), and LiH (+0.78 eV). Surprisingly, we find that the more affordable partitioned EOM-MP2 theory performs as well as EOM-CCSD. 

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