  The interlayer hybridization (IH) of van der Waals (vdW) materials is thought
to be mostly associated with the unignorable interlayer overlaps of
wavefunctions ($t$) in real space. Here, we develop a more fundamental
understanding of IH by introducing a new physical quantity, the IH admixture
ratio ${\alpha}$. Consequently, an exotic strategy of IH engineering in energy
space can be proposed, i.e., instead of changing t as commonly used, ${\alpha}$
can be effectively tuned in energy space by changing the onsite energy
difference ($2{\Delta}$) between neighboring-layer states. In practice, this is
feasible via reshaping the electrostatic potential of the surface by deposing a
dipolar overlayer, e.g., crystalline ice. Our first-principles calculations
unveil that IH engineering via adjusting $2{\Delta}$ can greatly tune
interlayer optical transitions in transition-metal dichalcogenide bilayers,
switch different types of Dirac surface states in Bi$_2$Se$_3$ thin films, and
control magnetic phase transition of charge density waves in 1H/1T-TaS$_2$
bilayers, opening new opportunities to govern the fundamental optoelectronic,
topological, and magnetic properties of vdW systems beyond the traditional
interlayer-distance or twisting engineering.
