On the Importance of Well-Defined Thermal Correlation Functions in Simulating Vibronic Spectra

Area of Science:

• Computational quantum chemistry
• Spectroscopy
• Theoretical chemistry

Background:

• Calculating thermal vibrational correlation functions presents challenges, particularly at low temperatures where indeterminate forms arise.
• The multivalued nature of vibrational correlation functions can lead to discontinuities and inaccurate vibronic spectra.

Purpose of the Study:

• To address the computational difficulties in determining thermal vibrational correlation functions.
• To develop a method for accurate calculations across temperature ranges, including the 0 K limit.
• To ensure the continuity of vibrational correlation functions for correct vibronic spectra.

Main Methods:

• Jointly considering the partition function and propagator in the harmonic approximation to resolve the 0/0 indeterminate form.
• Implementing a phase tracking procedure to maintain the continuity of the vibrational correlation function.
• Simulating UV-Vis absorption spectra of pentacene and benzene.

Main Results:

• A well-behaved expression for thermal vibrational correlation functions is derived, valid from high temperatures down to 0 K.
• The phase tracking procedure successfully recovers continuous correlation functions and accurate vibronic spectra.
• Simulated UV-Vis spectra for pentacene and benzene show good agreement with experimental data.

Conclusions:

• The developed methods overcome significant computational hurdles in thermal vibrational correlation function calculations.
• Accurate vibronic spectra can be obtained by ensuring the continuity of the correlation function using phase tracking.
• The approach provides a reliable tool for theoretical spectroscopy, validated by experimental comparisons.