  We have theoretically investigated the double core-hole (DCH) states of the
open-shell molecules and examined the possibility of DCH spectroscopy by means
of X-ray two-photon photoelectron spectroscopy (XTPPS). Energies of many DCH
states were obtained by the CASSCF calculations and the generalized intra- and
interatomic relaxation energies were evaluated. We show that XTPPS can extract
these quantities by the measurement of single and double core-hole ionization
potentials. We discuss the influence of chemical environment on the DCH states
with two holes at the same atomic site and at two different atomic sites.
