  The electronic and magnetic properties of BaTi$_{2}$As$_{2}$O have been
investigated using both the first-principles and analytical methods. The
full-potential linearized augmented plane-wave calculations show that the most
stable state is a site-selective antiferromagnetic (AFM) metal with a
$\text{2}\times \text{1}\times \text{1}$ magnetic unit cell containing two
nonmagnetic Ti atoms and two other Ti atoms with antiparallel moments. Further
analysis to Fermi surface and spin susceptibility shows that the site-selective
AFM ground state is driven by the Fermi surface nesting and the Coulomb
correlation. Meanwhile, the charge density distribution remains uniform,
suggesting that the phase transition at $200$ K in experiment is a
spin-density-wave (SDW) transition.
