Molecular dynamics simulations on [FePAH](+) pi-complexes of astrophysical interest: anharmonic infrared spectroscopy

In this article, classical Born-Oppenheimer molecular dynamics (MD) simulations in the microcanonical ensemble are performed on neutral and cationic polycyclic aromatic hydrocarbon (PAH) species, focusing on [FePAH](+) pi-complexes. Their anharmonic mid-infrared (mid-IR) spectra in the classical approximation are derived. This approach allows us to describe the influence of the energy of a system on its IR spectrum in terms of band-shifts and broadenings. The MD simulations are performed on a potential energy surface (PES) described at the self-consistent-charge density functional tight-binding level of theory. The PES is benchmarked on DFT calculations, showing the validity of the approach for complexes of Fe+ with PAHs larger than coronene (C24H12) that are of astrophysical interest. MD simulations at high temperature show the occurrence of the diffusion of the Fe cation on the surface of the PAH. It proceeds through the edge of the carbon skeleton which is the lowest energy pathway presenting barriers smaller than 1 eV. Although only qualitative information on the band broadenings can be obtained, we show that the dependence of the computed positions of the main bands of [C24H12](0/+) and [FeC24H12](+) p-complexes on temperature can be fit by linear laws. The spectral trends determined for [FeC24H12](+) are compared to those of N-substituted [C23NH12](+) and [SiC24H12](+) pi-complexes of astrophysical interest.