本项目将建立一种基于密度泛函理论计算结果的二阶非线性光学(NLO)晶体材料的理论设计方法。该方法采用改进的演化算法来预测晶体结构，并结合倍频系数等各种性质的理论模拟来考察晶体的光学性能。利用该方法，对理论设计的由不同类型无对称中心(NCS)结构单元所组成的"虚拟"晶体的构型和光学性能进行系统研究，探讨影响NCS结构单元空间排列方式的各种因素，从中寻找出NCS结构单元组装的规律和趋势；通过考察所设计的NLO晶体线性和二阶非线性光学性能与结构单元之间的内在关系，深入了解该类材料功能与结构和组成的敏感性，并筛选出可能具有优良性能的红外NLO晶体材料，为新型NLO材料的开发和可控制备提供可靠的理论依据和研究思路。

Based on the results of density functional theory calculations, we will propose a theoretical method to design the second order nonlinear optical (NLO) crystal materials. In this method, the crystal structure is predicted by the improved evolutionary algorithm, and various properties such as the second harmonic generation coefficients are simulated theoretically to investigate the optical performance of the crystal. Using this approach, the structure and optical properties of the "virtual" crystals that are constructed by different types of noncentrosymmetric (NCS) units are studied systematically. Through examining the influence factors of the spatial arrangement of the NCS units, we try to find the rules and trends for the integration of the NCS units. Meanwhile, to understand how the performance of the NLO crystal is sensitive to the structure and composition of the system, the relationship between linear or second order NLO properties and the structure of NCS unit is explored. According to the results of theoretical predictions, the new infrared NLO crystal materials with good performances may be achieved. Our project can provide reliable theoretical foundations and ideas to develop new NLO crystals, as well as to realize the controllable preparations of NLO materials.