  This work is aimed at the multi-configuration Hartree-Fock calculations of
the 6s ionization energies of lanthanides with configurations [Xe]4f^{N}6s^{2}.
Authors have used the ATSP MCHF version in which there are new codes for
calculation of spin-angular parts of matrix elements of the operators of
intraatomic interactions written on the basis of the methodology Gaigalas,
Rudzikas and Froese Fischer, based on the second quantization in coupled
tensorial form, the angular momentum theory in 3 spaces (orbital, spin and
quasispin), graphical technique of spin-angular integrations and reduced
coefficients (subcoefficients) of fractional parentage. This methodology allows
us to study the configurations with open f-shells without any restrictions,
thus providing the possibility to investigate heavy atoms and ions as well as
to obtain reasonably accurate values of spectroscopic data for such complex
many-electron systems.
