  Using first-principles calculations, we systematically study the adsorption
behaviors of molecular CO on the Be (0001) surface. By calculating the
potential energy surfaces, we find that CO molecularly adsorbs on the Be
surface with small energy barriers. The most stable adsorption state is found
to be the one at the surface fcc hollow site, and the one at the surface top
site is the adsorption state that has the smallest energy barrier. Based on
electronic structure analysis, we further reveal that during the molecular
adsorption, the $5\sigma$ bonding and $2\pi$ antibonding orbitals of CO
hybridize with $s$ and $p_z$ electronic states of Be, causing electrons to
transfer from CO to Be.
