The weak interlayer bonding in van der Waals layered materials results in low energy barriers for sliding and twisting, facilitating access to diverse metastable stacking configurations. Since stacking order governs crystal symmetry and physical properties, it serves as an extra degree of freedom for engineering material functionalities. Recently, the van der Waals compound ${\mathrm{SnP}}_2{\mathrm{Se}}_6$, which comprises atomically thin layers with intrinsic chiral symmetry, was shown to exhibit layer-independent second-harmonic generation [Nat. Commun. 14, 2521 (2023)]. Here, using first-principles calculations, we investigate 16 distinct stacking configurations to elucidate the relationship between interlayer arrangement and nonlinear optical response. We reveal that while all considered structures lack inversion symmetry, the second-harmonic generation response of ${\mathrm{SnP}}_2{\mathrm{Se}}_6$ varies drastically with stacking sequence. Specifically, translational shifts relative to the ground state structure enhance the second-order susceptibility, whereas mirrored (${\mathrm{A}}^{\mathrm{m}}$) or rotated (${\mathrm{A}}^{\mathrm{r}}$) layer arrangements tend to suppress it. Moreover, we observe that significant interlayer coupling induces a strong stacking-dependence in both electronic structure and linear optical properties of ${\mathrm{SnP}}_2{\mathrm{Se}}_6$. Our results provide insights into chiral van der Waals materials and establish stacking engineering as an effective strategy for tailoring the optoelectronic performance of ${\mathrm{SnP}}_2{\mathrm{Se}}_6$.

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• Electronic structure
• Optoelectronics
• Layered semiconductors
• First-principles calculations