Developing effective approaches to modulate the nonlinear optical (NLO) responses of materials is of great importance to design novel optoelectric devices. Recent studies have reported two distinct pressure-induced second-harmonic generation (SHG) modulation behaviors in $\mathrm{NbO}{\mathrm{I}}_2$, with one enhanced and the other suppressed. These observations have sparked considerable interest in the underlying physical mechanism. Using first-principles calculations, we have declared the structure evolution path of $\mathrm{NbO}{\mathrm{I}}_2$ under pressure, where spontaneous polarization decreases, and the Peierls distortion ratio holds. The SHG susceptibility exhibits a global reduction across all frequencies due to suppressive quantum geometric quantities. On the other hand, a series of artificial structures with different Peierls distortion ratios was designed to explain the SHG boost in experiments, which is attributed to modified electronic dispersion relations. In addition, by applying uniaxial strain to the polarization axis, our results show that optical transition probability overtakes the shift vector as the primary contributor in polarization-dependent SHG. Hence, we demonstrate tunable SHG response governed by distinct microscopic mechanisms in the $\mathrm{NbO}{\mathrm{I}}_2$ system. Our findings not only contribute to clarifying different pressure-induced SHG behaviors in $\mathrm{NbO}{\mathrm{I}}_2$ but also deepen our understanding of SHG evolution under external fields.

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• Pressure effects
• 2-dimensional systems
• Density functional calculations
• Optical second-harmonic generation