work function is calculated as the follow:
where Evac and EF are vacuum energy and Fermi energy,
respectively. The vacuum energy is estimated by averaging the
electrostatic potential over the middle plane of the vacuum layers
这个真空能好像需要自己找的,不过新版本可以直接计算出来,我可以给你个参考文献。First-principles study of the surface energy and work function of III-V semiconductor compounds PHYSICAL REVIEW B 75, 235322 2007�