  Recent studies have revealed that the interlayer interaction in
two-dimensional (2D) layered materials is not simply of van der Waals character
but could coexist with quasi-bonding character. Here we classify the interlayer
quasi-bonding interactions into two main categories (I: homo-occupancy
interaction, II: hetero-occupancy interaction) according to the occupancy of
the involved energy bands near the Fermi level. Then we investigate the
quasi-bonding-interaction-induced band structure evolution of several
representative 2D materials based on density functional theory calculations.
Further calculations confirm that this classification is applicable to generic
2D layered materials and provides a unified understanding of the total strength
of interlayer interaction, which is a synergetic effect of the van der Waals
attraction and the quasi-bonding interaction. The latter is stabilizing in main
category II and destabilizing in main category I. Thus, the total interlayer
interaction strength is relatively stronger in category II and weaker in
category I.
