Static vibrational polarizability of all-trans polyethylene and polysilane

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Abstract
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.