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1. 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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2. Building on the strengths of a double-hybrid density functional for excitation energies and inverted singlet-triplet energy gaps

   https://doi.org/10.1063/5.0133727  

   Curtis, Kevin ; Adeyiga, Olajumoke ; Suleiman, Olabisi ; Odoh, Samuel O. ( January 2023 , The Journal of Chemical Physics)

   It is demonstrated that a double hybrid density functional approximation, ωB88PTPSS, that incorporates equipartition of density functional theory and the non-local correlation, however with a meta-generalized gradient approximation correlation functional, as well as with the range-separated exchange of ωB2PLYP, provides accurate excitation energies for conventional systems, as well as correct prescription of negative singlet–triplet gaps for non-conventional systems with inverted gaps, without any necessity for parametric scaling of the same-spin and opposite-spin non-local correlation energies. Examined over “safe” excitations of the QUESTDB set, ωB88PTPSS performs quite well for open-shell systems, correctly and fairly accurately [relative to equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) reference] predicts negative gaps for 50 systems with inverted singlet–triplet gaps, and is one of the leading performers for intramolecular charge-transfer excitations and achieves near-second-order approximate coupled cluster (CC2) and second-order algebraic diagrammatic construction quality for the Q1 and Q2 subsets. Subsequently, we tested ωB88PTPSS on two sets of real-life examples from recent computational chemistry literature–the low energy bands of chlorophyll a (Chl a) and a set of thermally activated delayed fluorescence (TADF) systems. For Chl a, ωB88PTPSS qualitatively and quantitatively achieves DLPNO-STEOM-CCSD-level performance and provides excellent agreement with experiment. For TADF systems, ωB88PTPSS agrees quite well with spin-component-scaled CC2 (SCS-CC2) excitation energies, as well as experimental values, for the gaps between the S1 and T1 excited states. 

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3. 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/c9cp03695d  

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

   We report on the implementation of Dyson orbitals within the recently introduced frozen-core (fc) core–valence separated (CVS) equation-of-motion (EOM) coupled-cluster singles and doubles (CCSD) method, which enables efficient and reliable characterization of core-level states. The ionization potential (IP) variant of fc-CVS-EOM-CCSD, in which the EOM target states have one electron less than the reference, gives access to core-ionized states thus enabling modeling of X-ray photoelectron spectra (XPS) and its time-resolved variant (TR-XPS). Dyson orbitals are reduced quantities that can be interpreted as correlated states of the ejected/attached electron; they enter the expressions of various experimentally relevant quantities. In the context of photoelectron spectroscopy, Dyson orbitals can be used to estimate the strengths of photoionization transitions. We illustrate the utility of Dyson orbitals and fc-CVS-EOM-IP-CCSD by calculating XPS of the ground state of adenine and TR-XPS of the excited states of uracil. 

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4. State-specific solvation for restricted active space spin–flip (RAS-SF) wave functions based on the polarizable continuum formalism

   https://doi.org/10.1063/5.0091636  

   Alam, Bushra ; Jiang, Hanjie ; Zimmerman, Paul M. ; Herbert, John M. ( May 2022 , The Journal of Chemical Physics)

   The restricted active space spin–flip (RAS-SF) formalism is a particular form of single-reference configuration interaction that can describe some forms of strong correlation at a relatively low cost and which has recently been formulated for the description of charge-transfer excited states. Here, we introduce both equilibrium and nonequilibrium versions of a state-specific solvation correction for vertical transition energies computed using RAS-SF wave functions, based on the framework of a polarizable continuum model (PCM). Ground-state polarization is described using the solvent’s static dielectric constant and in the nonequilibrium solvation approach that polarization is modified upon vertical excitation using the solvent’s optical dielectric constant. Benchmark calculations are reported for well-studied models of photo-induced charge transfer, including naphthalene dimer, C 2 H 4 ⋯C 2 F 4 , pentacene dimer, and perylene diimide (PDI) dimer, several of which are important in organic photovoltaic applications. For the PDI dimer, we demonstrate that the charge-transfer character of the excited states is enhanced in the presence of a low-dielectric medium (static dielectric constant ɛ 0 = 3) as compared to a gas-phase calculation ( ɛ 0 = 1). This stabilizes mechanistic traps for singlet fission and helps to explain experimental singlet fission rates. We also examine the effects of nonequilibrium solvation on charge-separated states in an intramolecular singlet fission chromophore, where we demonstrate that the energetic ordering of the states changes as a function of solvent polarity. The RAS-SF + PCM methodology that is reported here provides a framework to study charge-separated states in solution and in photovoltaic materials. 

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5. Energy- and conformer-dependent excited-state relaxation of an E / Z photoswitchable thienyl-ethene

   https://doi.org/10.1039/c9cp01226e  

   Young, Jamie D. ; Honick, Chana R. ; Zhou, Jiawang ; Pitts, Cody R. ; Ghorbani, Fereshte ; Peters, Garvin M. ; Lectka, Thomas ; Tovar, John D. ; Bragg, Arthur E. ( July 2019 , Physical Chemistry Chemical Physics)

   Bis(bithienyl)-1,2-dicyanoethene (4TCE) is a photoswitch that operates via reversible E / Z photoisomerization following absorption of visible light. cis -to- trans photoisomerization of 4TCE requires excitation below 470 nm, is relatively inefficient (quantum yield < 5%) and occurs via the lowest-lying triplet. We present excitation-wavelength dependent (565–420 nm) transient absorption (TA) studies to probe the photophysics of cis -to- trans isomerization to identify sources of switching inefficiency. TA data reveals contributions from more than one switch conformer and relaxation cascades between multiple states. Fast (∼4 ps) and slow (∼40 ps) components of spectral dynamics observed at low excitation energies (>470 nm) are readily attributed to deactivation of two conformers; this assignment is supported by computed thermal populations and absorption strengths of two molecular geometries (P A and P B ) characterized by roughly parallel dipoles for the thiophenes on opposite sides of the ethene bond. Only the P B conformer is found to contribute to triplet population and the switching of cis -4TCE: high-energy excitation (<470 nm) of P B involves direct excitation to S 2 , relaxation from which prepares an ISC-active S 1 geometry (ISC QY 0.4–0.67, k ISC ∼ 1.6–2.6 × 10 −9 s −1 ) that is the gateway to triplet population and isomerization. We ascribe low cis -to- trans isomerization yield to excitation of the nonreactive P A conformer (75–85% loss) as well as loses along the P B S 2 → S 1 → T 1 cascade (10–20% loss). In contrast, electrocyclization is inhibited by the electronic character of the excited states, as well as a non-existent thermal population of a reactive “antiparallel” ring conformation. 

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