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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. Simulating X-ray photoelectron spectra with strong electron correlation using multireference algebraic diagrammatic construction theory

   https://doi.org/10.1039/d1cp05476g  

   de Moura, Carlos E. ; Sokolov, Alexander Yu. ( February 2022 , Physical Chemistry Chemical Physics)

   We present a new theoretical approach for the simulations of X-ray photoelectron spectra of strongly correlated molecular systems that combines multireference algebraic diagrammatic construction theory (MR-ADC) [ J. Chem. Phys. , 2018, 149 , 204113] with a core–valence separation (CVS) technique. The resulting CVS-MR-ADC approach has a low computational cost while overcoming many challenges of the conventional multireference theories associated with the calculations of excitations from inner-shell and core molecular orbitals. Our results demonstrate that the CVS-MR-ADC methods are as accurate as single-reference ADC approximations for predicting core ionization energies of weakly-correlated molecules, but are more accurate and reliable for systems with a multireference character, such as a stretched nitrogen molecule, ozone, and isomers of the benzyne diradical. We also highlight the importance of multireference effects for the description of core–hole screening that determines the relative spacing and order of peaks in the XPS spectra of strongly correlated systems. 

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4. Theory, implementation, and disappointing results for two-photon absorption cross sections within the doubly electron-attached equation-of-motion coupled-cluster framework

   https://doi.org/10.1063/5.0135052  

   Nanda, Kaushik D. ; Gulania, Sahil ; Krylov, Anna I. ( February 2023 , The Journal of Chemical Physics)

   The equation-of-motion coupled-cluster singles and doubles method with double electron attachment (EOM-DEA-CCSD) is capable of computing reliable energies, wave functions, and first-order properties of excited states in diradicals and polyenes that have a significant doubly excited character with respect to the ground state, without the need for including the computationally expensive triple excitations. Here, we extend the capabilities of the EOM-DEA-CCSD method to the calculations of a multiphoton property, two-photon absorption (2PA) cross sections. Closed-form expressions for the 2PA cross sections are derived within the expectation-value approach using response wave functions. We analyze the performance of this new implementation by comparing the EOM-DEA-CCSD energies and 2PA cross sections with those computed using the CC3 quadratic response theory approach. As benchmark systems, we consider transitions to the states with doubly excited character in twisted ethene and in polyenes, for which EOM-EE-CCSD (EOM-CCSD for excitation energies) performs poorly. The EOM-DEA-CCSD 2PA cross sections are comparable with the CC3 results for twisted ethene; however, the discrepancies between the two methods are large for hexatriene. The observed trends are explained by configurational analysis of the 2PA channels. 

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5. A combined experimental and computational study on the transition of the calcium isopropoxide radical as a candidate for direct laser cooling

   https://doi.org/10.1039/D1CP04107J  

   Telfah, Hamzeh ; Sharma, Ketan ; Paul, Anam C. ; Riyadh, S. M. ; Miller, Terry A. ; Liu, Jinjun ( April 2022 , Physical Chemistry Chemical Physics)

   Vibronically resolved laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down (CRD) spectra of the electronic transition of the calcium isopropoxide [CaOCH(CH 3 ) 2 ] radical have been obtained under jet-cooled conditions. An essentially constant energy separation of 68 cm −1 has been observed for the vibrational ground levels and all fundamental vibrational levels accessed in the LIF measurement. To simulate the experimental spectra and assign the recorded vibronic bands, Franck–Condon (FC) factors and vibrational branching ratios (VBRs) are predicted from vibrational modes and their frequencies calculated using the complete-active-space self-consistent field (CASSCF) and equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) methods. Combined with the calculated electronic transition energy, the computational results, especially those from the EOM-CCSD calculations, reproduced the experimental spectra with considerable accuracy. The experimental and computational results suggest that the FC matrix for the studied electronic transition is largely diagonal, but transitions from the vibrationless levels of the à state to the X̃-state levels of the CCC bending ( ν 14 and ν 15 ), CaO stretch ( ν 13 ), and CaOC asymmetric stretch ( ν 9 and ν 11 ) modes also have considerable intensities. Transitions to low-frequency in-plane [ ν 17 ( a ′)] and out-of-plane [ ν 30 ( a ′′)] CaOC bending modes were observed in the experimental LIF/DF spectra, the latter being FC-forbidden but induced by the pseudo-Jahn–Teller (pJT) effect. Both bending modes are coupled to the CaOC asymmetric stretch mode via the Duschinsky rotation, as demonstrated in the DF spectra obtained by pumping non-origin vibronic transitions. The pJT interaction also induces transitions to the ground-state vibrational level of the ν 10 ( a ′) mode, which has the CaOC bending character. Our combined experimental and computational results provide critical information for future direct laser cooling of the target molecule and other alkaline earth monoalkoxide radicals. 

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