  Designing systems with large magnetic anisotropy energy (MAE) is desirable
and critical for nanoscale magnetic devices. A recent breakthrough achieved the
theoretical limit of the MAE for 3$d$ transition metal atoms by placing a
single Co atom on a MgO(100) surface, a result not replicated by standard
first-principles simulations. Our study, incorporating Hubbard-$U$ correction
and spin-orbit coupling, successfully reproduces and explains the high MAE of a
Co adatom on a MgO (001) surface. We go further by exploring ways to enhance
MAE in 3d transition metal adatoms through different structural geometries of
3d--O molecules on MgO. One promising structure, with molecules perpendicular
to the surface, enhances MAE while reducing substrate interaction, minimizing
spin fluctuations, and boosting magnetic stability. Additionally, we
demonstrate significant control over MAE by precisely placing 3d--O molecules
on the substrate at the atomic level.
