Static vibrational polarizability of all-trans polyethylene and polysilane

The evolution with chain length of the vibrational contribution to the static linear polarizability of all-trans polyethylene and polysilane chains has been computed at the ab initio Hartree-Fock and MP2 levels within the double harmonic oscillator approximation. In polyethylene, the vibrational polarizability per unit cell is very small whereas in polysilane it amount to 50% of its electronic counterpart. The longitudinal component in polysilane mainly originates from one wagging mode and is a consequence of the delocalization due to σ-conjugation. Contrary to polysilane chains, where the local and non-local polarizations induced by the wagging motion of the hydrogen atoms are in phase, these contributions cancel each other in polyethylene chains and result in a very small vibrational polarizability.