A New and Efficient Equation-of-Motion Coupled-Cluster Framework for Core-Excited and Core-Ionized States

Abstract

We present a fully analytical implementation of the core-valence separation (CVS) scheme for the equation-of-motion (EOM) coupled-cluster singles and doubles (CCSD) method for calculations of core-level states. In the spirit of the original CVS approximation proposed by Cederbaum, Domcke and Schirmer, pure valence excitations are excluded from the EOM target space and the frozen-core approximation is imposed on the reference-state amplitudes and multipliers. This yields an efficient, robust, and accurate EOM-CCSD framework for calculations of excitation and ionization energies as well as state and transition properties (e.g., spectral intensities, natural transition and Dyson orbitals). The accuracy of the new scheme is improved relative to the results obtained applying the CVS only during the solution of the EOM eigenvalue equations. The errors in absolute excitation/ionization energies relative to the experimental reference data are of the order of 0.2{3.0 eV, depending on the K-edge considered and on the basis set used, and the shifts are systematic for each edge.<br>

Benchmark Calculations of K-Edge Ionization Energies for First-Row Elements Using Scalar-Relativistic Core–Valence-Separated Equation-of-Motion Coupled-Cluster Methods

“…This leads to the same scheme for the solution of the excited-state secular equation adopted here together with the frozen-core approximation for the ground state calculation. 84 The purpose of the current study is to benchmark the performance of CVS-EOMIP-CC methods for calculations of core ionization energies. In particular, we aim to understand how the results converge with respect to the excitation rank and how the near-CVS-FCI limit compares with experiment.…”

Excited states via coupled cluster theory without equation-of-motion methods: Seeking higher roots with application to doubly excited states and double core hole states

“…The removal of unoccupied core orbitals is similar in spirit to the CVS [13][14][15][16][17] treatment in EOM-CC and it explicitly prevents the CC wavefunction from collapsing to the ground state of the same number of particles. In our case, a double excitation from HOMO to the unoccupied core orbitals would yield much lower energy than the desired core-ionized state.…”

“…However, one has to obtain a large number of eigenvectors to cover the energy range for core ionizations which can be very time-consuming for an O(N 5 ) method. There are tricks to remedy this problem to an extent via core valence separation (CVS) [13][14][15][16][17] , but it does not solve the inherent drawbacks of EOM-IP-CCSD. In other words, CVS-EOM-IP-CCSD fails when EOM-IP-CCSD fails.…”

Orbital Optimized Density Functional Theory for Electronic Excited States

“…Figure 5. Comparison of computed (recoupled OO-DFT with SCAN and fc-CVS-EOM-CCSD,109 with the aug-cc-pCVTZ basis) spectra with experiment, without any empirical translation of spectra. The computed peaks were broadened by a Voigt profile with a Gaussian standard deviation of 0.2 eV and Lorentzian γ = 0.121 eV.…”