A relativistic formalism to construct the Hamiltonian for a general N-electron neutral atom with a spin-1/2 nucleus and placed in a homogeneous magnetic field [J. Chem. Phys. 123 064101 (2005)] is discussed here. Projected Coulomb and Breit interactions are taken to describe the inter-particle interaction through second order in fine structure constant. The center of mass motion is separated from the movement of the neutral (N+1) fermion system. Using a series of suitable unitary transformations, the Hamiltonian can be brought into a separable form. The correct atomic Hamiltonian in the non-relativistic limit is also discussed. The treatment can be extended to the case of a molecule containing atoms of only spin-1/2 nuclei