The interlayer quasi-chemical-bonding (QCB) interactions of two-dimensional layered materials advance the research field of interlayer engineering and cause interlayer charge density redistributions (ICDRs). The ICDRs have been reported experimentally and theoretically, showing different redistributions, e.g., accumulation, depletion, or a more complicated behavior. The underlying mechanism for the different ICDRs remains to be elucidated. In the current work, via a systematic theoretical study of the ICDRs of transition metal dichalcogenides with different numbers of $d$ electrons filling ($d^0 {\mathrm{TiS}}_2, d^1 {\mathrm{NbS}}_2$, and $d^2 {\mathrm{MoS}}_2$) in the $T$ and $H$ phases, we reveal three mechanisms based on the coexistence of different types of interlayer QCB interactions. Mechanism (1) is from a competition between two types of interlayer interactions: namely, the interlayer interaction between fully occupied energy levels (in short: o-o interaction) depletes electrons in the overlap region while that between occupied and empty levels (o-e interaction) promotes electron accumulation; the competition between them leads to the $d^0 {\mathrm{TiS}}_2$ tending to electron accumulation in the $T$ phase more than in the $H$ phase. Mechanism (2), the interlayer interaction between half-filled levels (h-h interaction), promotes the electron accumulation of $d^1 {\mathrm{NbS}}_2$. Mechanism (3), the interlayer interaction of multiple filled levels of $d^2 {\mathrm{MoS}}_2$ (namely, the multilevel o-o interaction), leads to a more complicated ICDR. The current study provides a unified understanding of the different ICDRs of van der Waals materials and paves the way for further exploration of their electronic properties and applications.

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• Chemical bonding
• Electronic structure
• Electronic structure of atoms & molecules
• Layered semiconductors
• Transition metal dichalcogenides
• Density functional theory