Probing Basis Set Requirements for Calculating Core Ionization and Core Excitation Spectra Using Correlated Wave Function Methods

We
investigate the basis set requirements for the accurate calculation
of core excitations and core ionizations using correlated wave functions
of coupled cluster type and linear response methods for describing
the excitation. When a core excitation is described as an energy difference
calculated using density functional theory, the basis set can be tailored
to provide a balanced description of the reference- and excited-hole
states. When the core excitation process is described by coupled cluster
linear response methods, however, the basis set requirements are somewhat
different. A systematic study of the sensitivity of the result to
the basis set parameters suggests that a relatively large set of s-
and p-type basis functions in combination with a careful selection
of valence and core polarization functions is required. Based on these
results, we propose a hierarchical sequence of basis sets, denoted
ccX-nZ (n = D, T, Q, 5) for the
atoms B–Ne, which are suitable for the calculation of core
excitations by the correlated wave function linear response and equation-of-motion
methods. The ccX-nZ series provides lower basis set
errors for a given cardinal number or number of basis functions than
other existing basis sets. For large systems, the ccX-nZ basis sets can be combined with the standard basis sets by placing
the ccX-nZ only on the atoms where core excitations
are of interest, but the accuracy of such mixed basis sets appears
to be system-dependent.