Combining ab initio and ab initio molecular dynamics simulations to predict the complex refractive indices of organic polymers.

Abstract

Organic polymers have attracted widespread interest in various fields ranging from optic and optoelectronic devices to optical system design owing to their light weight, high machinability, excellent thermal performance and reasonable costs. The complex refractive index is an inherent property of organic polymers and directly affects the accuracy of optical system simulation. This study introduces a theoretical protocol to accurately predict the complex refractive indices of organic polymers in the 0-5000 cm-1 region for guiding the discovery and design of high-refractive index materials. In the proposed protocol, we computed the refractive indices of polymers with different monomer units using ab initio calculated static polarizability and mass density obtained by classical isothermal-isobaric ensemble simulations based on the Lorentz-Lorenz equation; we proposed a "Polymer Polarizability Fragment Segmentation" method to extrapolate the polarizabilities of polymers with longer chain lengths; meanwhile, the imaginary part of the dielectric functions of the polymers was calculated using the ab initio molecular dynamics (AIMD) method, and the real part of the dielectric functions was obtained using the Kramers-Kronig relation. We calculated the complex refractive indices of four commonly used organic polymers, i.e. polyethylene, polyvinyl chloride, polyvinyl alcohol and polylactic acid, to demonstrate the performance of the theoretical protocol. The approach combining ab initio and AIMD simulations is effective and economical to predict the complex refractive indices of organic polymers and other organic materials.