  The incorporation of the Breit interaction in atomic and molecular
calculations is discussed in the framework of four-component all-electron and
two-component relativistic effective core potential (RECP)formalisms.
Contributions of the Breit interaction between different core and valence
shells are studied in the Dirac-Fock approximation for the uranium, plutonium,
ekathallium (Z=113), ekalead (Z=114) and other heavy atoms. It is shown that
the two-electron Breit effects between the valence electrons can be neglected
for "chemical accuracy" (1 kcal/mol) of calculation of spectroscopic properties
of systems containing superheavy elements and actinides whereas large core-core
and core-valence Breit contributions can be efficiently described by
one-electron RECP operators. Different versions of the generalized RECPs with
the Breit interaction taken into account are constructed for uranium and
plutonium and tested in comparison with the corresponding all-electron
four-component calculations.
