  The issue of orbital relaxation in computational core-hole spectroscopy,
specifically x-ray absorption, has been a major problem for methods such as
equation-of-motion coupled cluster with singles and doubles (EOM-CCSD). The
transition-potential coupled cluster (TP-CC) method is utilized to address this
problem by including an explicit treatment of orbital relaxation via the use of
reference orbitals with a fractional core occupation number. The value of the
fractional occupation parameter $\lambda$ was optimized for both TP-CCSD and
XTP-CCSD methods in an element-specific manner due to the differences in atomic
charge and energy scale. Additionally, TP-CCSD calculations using the optimized
parameters were performed for the K-edge absorption spectra of gas-phase
adenine and thymine. TP-CCSD reproduces the valence region well and requires
smaller overall energy shifts in comparison to EOM-CCSD, while also improving
on the relative position and intensities of several absorption peaks.
