Modern simulation methods for vibrational and electronic spectroscopy

Thesis

Spectroscopy offers a convenient way to collect information about molecular systems in a non-invasive way. Vibrational and electronic spectra may both be simulated by calculating response functions and Fourier transforming them to obtain spectra.

Vibrational spectra are often calculated using path integral methods, which can approximately include nuclear quantum effects in the linear response function at a linearly scaling computational cost. This thesis introduces a generalisation of f-QCMD, a promising recently developed method, to the condensed phase, enabling the calculation of the vibrational spectrum of liquid water and ice. The thesis then uses f-QCMD to analyse polariton spectra, which occur when a molecular system is placed in a microcavity. The vibrational spectrum gives a uniquely uncontroversial (compared to other properties like reaction rates) insight into cavity effects. In fact, a simple harmonic oscillator model which takes only the cavity-free spectrum and the geometry of the cavity as input can quantitatively predict cavity molecular dynamics results, implying that the cavity simply acts as an optical filter on the linear vibrational spectrum.

The thesis then moves on to the simulation of two-dimensional electronic spectra. It introduces the mean field method ‘equatorial Ehrenfest’ that qualitatively captures the two-dimensional electronic spectra of Frenkel exciton models, which emulate light harvesting complexes. This approach is inspired by the previously proposed polar Ehrenfest approach, which is 32 times more expensive and less accurate than equatorial Ehrenfest. Substituting Ehrenfest with a more accurate dynamics method only moderately improves the quantitative agreement with exact results, at increased computational cost.

Authors: Lieberherr, AZ

• DOI: 10.5287/ora-eozzoaxk0
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: quantum dynamics; vibrational spectroscopy; electronic spectroscopy; path integrals; nonadiabatic dynamics; nuclear quantum effects; molecular dynamics
• Subjects: Chemistry, Physical and theoretical