SEMICLASSICAL VIBRATIONAL SPECTROSCOPY BEYOND POWER SPECTRA

Infrared (IR) spectroscopy is a fundamental technique for probing molecular structure, dynamics, and intermolecular interactions. However, the increasing spectral complexity of medium- and large-sized systems makes theoretical support essential for reliable spectral assignment. Semiclassical (SC) molecular dynamics offers an effective compromise between accuracy and computational cost, incorporating key quantum effects through the propagation of classical trajectories. Traditionally, SC vibrational spectroscopy has focused on the calculation of power spectra, which provide vibrational energy levels but do not directly reproduce experimental IR intensities. This Ph.D. thesis presents the development of a new semiclassical approach for the direct calculation of IR absorption spectra (SC IR) within the semiclassical initial value representation framework. The method enables the simultaneous prediction of vibrational frequencies and physically meaningful transition intensities, producing signals exclusively for IR-active modes and allowing direct comparison with experimental spectra. After validation on model systems and small molecules, an extended formulation (e-SC IR) based on mode-by-mode simulations is introduced to treat larger molecular systems with improved efficiency. These developments extend semiclassical vibrational spectroscopy beyond power spectra toward a more realistic and experimentally oriented description of IR absorption spectra.