Finite-temperature infrared spectroscopy of polycyclic aromatic hydrocarbon molecules. II. Principal mode analysis and self-consistent phonons.

Following previous work [F. Calvo et al. J. Chem. Phys. 132, 124308 (2010)], infrared spectra of several polycyclic aromatic hydrocarbon molecules are simulated with classical and quantum molecular dynamics trajectories. The interactions are modeled using a tight-binding potential energy surface and quantum delocalization is accounted for using the partially adiabatic centroid and ring-polymer molecular dynamics frameworks, both built upon the path-integral representation. The spectra obtained directly by Fourier transformation of the dipole moment autocorrelation function are here compared with several quasiharmonic approximations that provide additional information about the vibrational modes. A principal mode analysis (PMA) is carried out from the covariance matrix of atomic displacements in classical and quantum trajectories. The method systematically overestimates the line shifts due to anharmonicities, except in the power spectra of atomic displacements, and is not robust in predicting IR intensities for such large molecules. Alternatively, effective normal modes have also been determined by adapting the self-consistent phonon (SCP) theory of condensed matter physics to the present tight-binding model, in both classical and quantum mechanical descriptions. The SCP approximation turns out as semiquantitative in estimating the redshift of tight stretching modes, and performs better for classical systems. More problematic, it predicts that many low- or medium-frequency modes should be blueshifted, in contradiction with the molecular dynamics results. The sets of anharmonic normal modes extracted from the PMA and SCP approaches reveal important mixings within the tightest C-H and C-C stretching modes, which are also manifested on the corresponding power spectra.