posted on 2019-08-14, 18:34authored byXuechen Zheng, Lan Cheng
A thorough
study of the performance of delta-coupled-cluster (ΔCC)
methods for calculations of core-ionization energies for elements
of the first long row of the periodic table is reported. Inspired
by the core–valence separation (CVS) scheme in response theories,
a simple CVS scheme of excluding the vacant core orbital from the
CC treatment has been adopted to solve the convergence problem of
the CC equations for core-ionized states. Dynamic correlation effects
have been shown to make important contributions to the computed core-ionization
energies, especially to chemical shifts of these quantities. The maximum
absolute error (MaxAE) and standard deviation (SD) of delta-Hartree–Fock
results for chemical shifts of core-ionization energies with respect
to the corresponding experimental values amount to more than 1.7 and
0.6 eV, respectively. In contrast, the inclusion of electron correlation
in ΔCC singles and doubles augmented with a noniterative triples
correction [ΔCCSD(T)] method significantly reduces the corresponding
deviations to around 0.3 and 0.1 eV. With the consideration of basis
set effects and the corrections to the CVS approximation, ΔCCSD(T)
has been shown to provide highly accurate results for absolute values
of core-ionization energies, with a MaxAE of 0.22 eV and SD of 0.13
eV. To further demonstrate the usefulness of ΔCCSD(T), calculations
of carbon K-edge ionization energies of ethyl trifluoroacetate, a
molecule of significant interest to the study of X-ray spectroscopy
and dynamics, are reported.