  Calculations of transition energies between low-lying states of mercury atom
are performed in the frame of combined CI/MBPT2 method. Results of all-electron
relativistic calculations (using the Dirac-Coulomb Hamiltonian) are compared
with experimental data and results of other four-component calculations. The
results of the RECP calculations are compared with the corresponding
all-electron results in order to estimate accuracy of different RECPs.
Contributions from correlations in different shells to the calculated
excitation energies as well as effects of basis set truncation at different
orbital angular momenta, nuclear models, errors in gaussian approximation of
the GRECP components are reported. Analysis of the obtained results shows that
at least 34 external electrons of mercury atom should be correlated and the
one-electron basis set should contain up to h angular momentum functions in
order to reach a reliable agreement with experimental data within 200 1/cm. It
is concluded that correlations of the 4f electrons can be efficiently taken
into account for 20 electron GRECP at the generation stage.
