CdSiP2(CSP)crystal is expected as one of the successful crystals with well-known chalcopyrite ABⅢCⅥ2type in the field of nonlinear optical applications,considering its apparent birefringence,high nonlinear optical coefficients and good phase-matching property.However,the defects-affecting slight absorption losses around 1.34 μm and 1.78 μm in CSP may seriously reduce the optical damage threshold and affect the conversion efficiency in making promising optical frequency conversion devices.In this study,the calculations were performed with Vienna Ab Initio Simulation Package(VASP)using Heyd,Scuseria,and Ernzerh(HSE)to identify the effects of various defects on its absorption properties.Different defects and dopants in CSP crystal were calculated and compared according to the specific chemical environments in the experiments.The results show that the point defects of VCd2-,SiCd2+ and VSi4-,which can be spontaneously formed,are dominating in the Cd-poor environment.The combination of SiCd2+and VCd2-defects is the most favorable cluster in Cd-poor case because of its relatively low formation energy.Furthermore,the antisite defect SiCd is found to be responsible for the main absorption peaks at 1.34 and 1.78 μm in the experimental spectra,while other defects and clusters,such as the dopants Fe and Mn,also contribute to the losts.Our results intend to provide a guideline for adjusting the optical absorption in CSP by modifying its defects.