Comment on "Quasirelativistic theory equivalent to fully relativistic theory" [J. Chem. Phys. 123, 241102 (2005)].

The connection between the exact quasirelativistic approach developed in the title reference [W. Kutzelnigg and W. Liu, J. Chem. Phys. 123, 241102 (2005)] and the method of elimination of the small component in matrix form developed previously by Dyall is explicitly worked out. An equation that links Hermitian and non-Hermitian formulations of the exact quasirelativistic theory is derived. Besides establishing a kinship between the existing formulations, the proposed equation can be employed for the derivation of new formulations of the exact quasirelativistic theory.