Cherry-picking resolvents: A general strategy for convergent coupled-cluster damped response calculations of core-level spectra

K.D. Nanda and A.I. Krylov
J. Chem. Phys.  153, 141104 (2020)

Damped linear response calculations within the equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) framework usually diverge in the X-ray regime. This divergent behavior stems from the valence ionization continuum in which the X-ray response states are embedded. Here, we introduce a general strategy for removing the continuum from the response manifold while preserving the important spectral properties of the model Hamiltonian. The strategy is based on decoupling the core and valence Fock spaces using the core--valence separation (CVS) scheme, followed by separate (approximate) treatment of the core and valence resolvents. We illustrate this approach with the calculations of resonant inelastic X-ray scattering spectra of benzene and \emph{para}-nitroaniline using EOM-CCSD wave functions and several choices of resolvents, which differ in their treatment of the valence manifold. The method shows robust convergence and extends the previously introduced CVS-EOM-CCSD RIXS scheme to systems for which valence contributions to the total cross section are important, such as the push--pull chromophores with charge-transfer states.

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