Length Dependence of Ionization Potentials of Trans-Acetylenes: Internally-Consistent DFT/GW Approach

TL;DR

This study investigates how ionization potentials of trans-acetylenes change with chain length, using a novel internally-consistent DFT/GW approach to accurately predict their electronic properties from short oligomers to infinite polymer.

Contribution

It introduces an internally-consistent scheme to adjust hybrid functional parameters for better quasiparticle energy predictions in conjugated molecules.

Findings

IP converges smoothly to the polymer value with increasing chain length.

The system-dependent exchange parameter $\alpha^{ic}$ converges with oligomer length.

The method accurately reproduces experimental IP data for short oligomers.

Abstract

We follow the evolution of the Ionization Potential (IP) for the paradigmatic quasi-one-dimensional trans-acetylene family of conjugated molecules, from short to long oligomers and to the infinite polymer trans-poly-acetylene (TPA). Our results for short oligomers are very close to experimental available data. We find that the IP varies with oligomer length and converges to the given value for TPA with a smooth, coupled inverse-length-exponential behavior. Our prediction is based on an "internally-consistent" scheme to adjust the exchange mixing parameter $\alpha$ of the PBEh hybrid density functional, so as to obtain a description of the electronic structure consistent with the quasiparticle approximation for the IP. This is achieved by demanding that the corresponding quasiparticle correction, in the GW@PBEh approximation, vanishes for the IP when evaluated at PBEh($\alpha^{ic}$). We find that $\alpha^{ic}$ is also system-dependent and converges with increasing oligomer length, allowing to capture the dependence of IP and other electronic properties.