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Calculations for the Linear and Nonlinear Optical Coefficients of ZCTC Crystals Chao Feng1, Guiwu Lu*, 1, Xinqiang Wang2, Peng Zhang1, Guanggang Zhou1, Hongwang Yang1, Yuqiu Jiao1 1 Department of Mathematics and Physics, China University of Petroleum, Beijing 102249, China 2 Institute of Crystal Materials and State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, China Received ZZZ, revised ZZZ, accepted ZZZPublished online ZZZ Keywords Density functional theory (DFT); second harmonic generation (SHG) coefficients; second-order nonlinear optics; electronic structure The structural, electronic, linear and nonlinear optical properties of ZnCd(SCN)4 (ZCTC) single crystal were studied by using quantum-mechanical calculations based on the density functional theory (DFT) and pseudopotential method. The optimized lattice constant can be compared with the experimental values when the effects of temperature are considered. Combining with partial density of states of ZCTC crystal, it was found that C2p, N2p and S3p orbitals occur obvious hybrid, which is the main source of the second-order nonlinear optical effect of ZCTC crystal. Then DFT has been used to calculate the hyperpolarizability of ZCTC single crystal, from which the second harmonic generation (SHG) coefficients was calculated using ionic group theory. The calculated SHG coefficients are d14=1.08~1.93 pm/V and d15=4.01~4.79 pm/V. Both of them are in agreement with the tendencies shown by the experimental results (d14=3.2 pm/V and d15=7.6 pm/V). It was also found that the greatest contribution for SHG of ZCTC crystal is from SCN anionic group. Copyright line will be provided by the publisher 1 Introduction Currently, second-order nonlinear optics continues to be a topical area of research because of its tremendous potentials in optical electronic applications such as electro-optical (EO) switches, ultrafast devices for information processing, storage, computing and et al. [1-5]. Zinc