Recently ternary metal halide HgBrI has been reported as an infrared nonlinear optical crystal (Wu et al., 2013) [1]. In this Letter, the binary metal halide HgI2 is computationally predicted to exhibit the static second-order coefficient seven times larger than that of HgBrI. The DFT studies on the structural parameters, optical band-gaps and optical properties of metal halides HgBr2, HgBrI and HgI2 are in agreement with the available experimental results and strongly suggest that HgI2 is a promising mid-to-far infrared nonlinear optical crystal. The results benefit to the further development of novel infrared nonlinear optical crystals in metal halides. (C) 2014 Elsevier B.V. All rights reserved.