The calculations of band structures using density-functional theory were conducted with the VASP code. We employed the VASP 5.4 PBE projector-augmented wave (PAW) pseudo-potentials and the optB88-vdW functions for band structure calculations. The total valence electrons explicitly treated in the DFT calculations consist of Nb 4p 6 5s 1 4d 4 , Se 4s 2 4p 4 , and I 5s 2 5p 5 , which are described by a plane-wave basis set with a cutoff energy of 550 eV. We performed self-consistent calculations with spin-orbit coupling on the primitive P 1 structure, allowing atom position relaxation, using all-acute angles to be compatible with the standard k-path, similar to the cell definition in the Open Quantum Materials Database: 48,49 a = 6.993 Å, b = 7.136 Å, c = 7.768 Å; α = 67.994 • , β = 66.801 • , and γ = 59.462 • . To simulate the monolayer structure, the vacuum spacing between the Van der Waals layers is increased to 18 Å, and the input k-path of the two-dimensional 2D Brillouin zone for the single layer calculation was selected using the VASPKIT code with the high symmetry points being: Γ (0, 0, 0), X (0.5, 0, 0), H1 (0.66, 0.32, 0), C (0.5, 0.5, 0), H (0.34, 0.68, 0), and Y (0, 0.5, 0).