The static linear and nonlinear optical properties of the σ-conjugated polymer α-t-Bu-ω-CN-poly(methylphenyl)silane (PMS) are studied at the Coupled-Perturbed Hartree-Fock (CPHF) level with 6-31+G(d) basis set. The calculated results reveal that the static first hyperpolarizabilities of this system increase with the main chain length and have a good agreement with experiments. The (hyper)polarizabilities per unit cell have been extrapolated to infinite chain limit and a comparison is made to those of polysilane and polyacetylene (PA). Besides, other structural properties depending on the σ-conjugated Si–Si skeleton length are investigated as well. Electron correlation effect is estimated and it turns out that the MP2 static first hyperpolarizability is about times larger than the corresponding CPHF value for the polymer with . 1. Introduction In recent years, polysilanes have attracted increasingly extensive attention due to their unique physical and chemical properties resulting from σ-electrons delocalized along the silicon backbone. In this regard, they have resulted in a variety of technological applications, such as conductor and semiconductor [1], photoconductive materials [2], organic multilayer, light emitting diodes (LEDs) [3], high-density optical data storage materials [4], electro luminescence (EL) devices [5, 6], and nonlinear optical (NLO) materials [7, 8]. The polysilanes represent a completely new class of potentially interesting nonlinear optical (NLO) materials, and in contrast to the classical π-conjugated polymers, their physical, chemical, and optical properties show significant differences. Polysilanes are linear polymers of silicon, and the σ-electrons of the polymer backbone are delocalized. This σ-conjugation gives rise to electronic properties that allow for possible applications as electroluminescent, nonlinear, optical, lithographic, and semiconductor materials. The σ-electrons delocalization associated with the phenomenon, in which the silicon backbone itself is a chromophore, greatly influences the optical properties of polysilanes. Therefore, in spite of saturation of all the Si atoms on the main chain, polysilanes have strong absorption in the UV in the region of 250–400？nm, which is attributed to the σ-σ* transition or the σ3d transition and relevant to the configuration and length of the Si–Si backbone. As NLO materials, in comparison with π-conjugated carbon-backbone polymers, polysilanes own many unique advantages; for example, they have optical transparency in the visible spectrum, their UV absorption peak can be
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