Molecular inner-shell photoabsorption/photoionization cross sections at core-valence-separated coupled cluster level: Theory and examples

Abstract

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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

“…Building on the (EOM-)CC damped response implementation reported by Faber and Coriani 24 , here we present a novel damped response implementation for computing RIXS cross sections with a variant of the EOM-CC singles and doubles method for electronic excitation [35][36][37]67,68 (EOM-EE-CCSD) that provides converged response solutions by judicious exploitation of the excitation manifold. Our strategy to achieve robust convergence of the response equations in RIXS calculations is to employ the corevalence separation (CVS) scheme originally proposed by Cederbaum et al 69 This approach has been employed to compute resolved core-excited and core-ionized states within the framework of EOM-CC and CC-LR response theory 19,64,70,71 , but has not yet been employed for multiphoton X-ray processes such as RIXS. The CVS scheme projects out the valence ionization continuum and enables the resolution of core-excited (real or response) states.…”

TD-DFT simulations of K-edge resonant inelastic X-ray scattering within the restricted subspace approximation

“…Finally, the eigenvalue problem is routinely solved employing implementations based on the iterative Davidson's algorithm. 32 In order to reach the core-excited states, typically, one projects eqn (8) onto an orbital subspace, [33][34][35] this procedure is also referred to as the core-valence separation (CVS) approximation, 18,36 this approach entails the restriction of the indices i, j in eqn (9) to include only the core-hole orbitals, while the indices a, b run over the full set of virtual orbitals. The main motivation for this constraint is reducing the required number of roots that are needed to reach the core-excited states.…”

Accurate Description of Photoionization Dynamical Parameters

“…Also, large basis sets are required and the energy resolution is quite limited. Even then, these methods have proven quite successful in combination with highly correlated electronic structure methods such as linear response coupled cluster [28][29][30][31] and algebraic diagrammatic construction (ADC). [32][33][34] The simplest approach to approximate the continuum is with plane waves (PW) or…”