  The electronic spectra of light one-electron quasi-molecular compounds
H-H$^+$, He$^+$-He$^2+$ and He$^+$-H$^+$ are analyzed. To this end, the
two-center Dirac equation is solved by the dual-kinetically balanced
finite-basis-set method for axially symmetric systems termed as A-DKB. This
method allows a complete relativistic consideration of these systems at fixed
internuclear distances. A comparison of the obtained results with the
nonrelativistic and relativistic calculations presented in the literature is
performed. The advantages and disadvantages of the approach are discussed in
details.
