The geometry structures of squaraine derivatives substituted by benzo five-membered ring moiety were optimized and electronic structures were calculated with HF/6-31G* method. Based on the optimized structures, the electronic spectra were obtained by the ZINDO method. The second-order nonlinear optical coefficients β_μ were calculated by ab initio method at CPHF/6-31G* level and finite field method at FF/AM1, FF/PM3, FF/MNDO and FF/MINDO3 levels, respectively. The relationship among geometry structure, electronic structure, frontier MO energy and β_μ was discussed in order to offer a theoretical instruction for designing organic nonlinear optical materials with good properties, and a systematic comparison between different calculated methods was carried out.

Key words: benzo five-membered ring, finite field method, second-order nonlinear optical coefficient, coupled perturbed Hartree-Fock (CPHF) method, squaraine