Ab initio simulation of UV/vis absorption spectra for atmospheric modeling: method design for medium-sized molecules

Area of Science:

• Computational Chemistry
• Theoretical Chemistry
• Molecular Spectroscopy

Background:

• Accurate calculation of absorption cross-sections is crucial for understanding molecular photodissociation.
• Previous methods often struggle with computational cost when accounting for multiple vibrational degrees of freedom.

Purpose of the Study:

• To develop an efficient computational procedure for accurate absorption cross-sections of dissociative excited states.
• To enable the study of medium-sized molecules with multiple photodissociation pathways.

Main Methods:

• Solving the vibrational Hamiltonian using a discrete variable representation (DVR) framework.
• Employing electron-correlated methods, specifically Equation-of-Motion Coupled-Cluster with Singles and Doubles (EOM-CCSD) and Similarity Transformed Equation-of-Motion Coupled-Cluster (STEOM-CCSD).
• Utilizing the ACESII quantum chemistry package.

Main Results:

• The presented procedure effectively approximates numerous vibrational degrees of freedom with minimal computational expense.
• Accurate absorption cross-sections for dissociative excited states can be obtained.
• A theoretical absorption cross-section for NaOH was successfully computed as a demonstration.

Conclusions:

• The novel computational approach offers a balance between accuracy and efficiency for studying molecular photodissociation.
• This method is applicable to medium-sized molecules with complex photodissociation dynamics.
• The procedure provides valuable insights into excited-state properties and photochemical processes.