The vibrational (gamma(L)(upsilon)) and electronic (gamma(L)(e)) longitudinal second hyperpolarizabilities of increasingly large polysilane chains are determined at the Hartree-Fock 6-31G level by adopting both the double harmonic oscillator approximation and the infinite optical frequency finite field relaxation procedure. The relative importance of the electronic, Raman, infrared/hyperRaman and lowest-order anharmonicity contributions to the second hyperpolarizability is evaluated for the most common nonlinear optical (NLO) processes. At the double harmonic oscillator level of approximation the most contributing vibrational normal modes to gamma(L)(upsilon) are characterized as a function of the polysilane chain length. Comparisons with experimental and other theoretical studies are carried out in what concerns the infrared and Raman vibrational spectra as well as the NLO properties of various oligosilanes and polysilanes.