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

Abstract

We present a novel approach with robust convergence of the response equations for computing resonant inelastic X-ray scattering (RIXS) cross sections within the equation-of-motion coupled-cluster (EOM-CC) framework.

Resonant Inelastic X-Ray Scattering Reveals Hidden Local Transitions of the Aqueous OH Radical

“…The CT transitions in the RIXS spectrum are suppressed because the compact shape of the core 1s O orbital results in poor overlap with the lone pairs of neighboring waters. We confirm this by ab initio RIXS calculations using a new electronic structure method [22,23] based on the equation-of-motion coupled-cluster (EOM-CC) theory. These calculations also reproduce the relative RIXS line intensities, positions, and widths for the elastic and inelastic peaks of OHðaqÞ and OH − ðaqÞ.…”

“…Rather than arbitrarily truncating the sum over states, we replace all ϵ n s with a phenomenological damping factor ϵ and use damped response theory to convert the KHD expression into a numerically tractable closed form [23,37,40]. Robust convergence of the auxiliary response equations is achieved by using CVS within the damped response domain [23]. The resulting method combines rigorous treatment of RIXS cross sections and high-level description of electron correlation.…”

Photon-In/Photon-Out X-ray Free-Electron Laser Studies of Radiolysis

“…Here, advances in equation-of-motion coupled-cluster (EOM-CC) approaches [77,78] have allowed a systematic convergence after application of the core-valence separation scheme [79] within the EOM-CC framework [80]. This general approach has allowed EOM-CC-CVS to achieve improved accuracy for single photon transitions [40,81] and expand to two-photon RIXS calculations [82]. Beyond high-level calculations of electronic structure, capturing coreexcited state dynamics poses a further challenge that has been largely pursued with simpler electronic structure methods [72,83,84].…”

Feshbach-Fano Approach for Calculation of Auger Decay Rates Using Equation-of-Motion Coupled-Cluster Wave Functions: Theory and Implementation

“…Eqs. (39) and (40) use left and right Dyson functions, which are not simple conjugates of each other in non-Hermitian frameworks, such as CC/EOM-CC. In the case of Hermitian approaches, these equations reduce to contain the absolute squares of one amplitude.…”

“…Thus, Slater determinants, in which at least one core orbital is active, form the bound domain, which can only couple to the continuum by pure valence excited determinants. This is exploited in the core-valence separation (CVS) [33] scheme commonly used to adapt standard electronic structure methods for treating core-level states [34][35][36][37][38][39][40][41][42][43]. The CVS ansatz decouples core-excited and core-ionized states from the valence continua, essentially acting as the Feshbach Q projector.…”