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

Abstract

A procedure is presented to obtain accurate absorption cross-sections for dissociative excited states. The focus is the ability to approximate many vibrational degrees of freedom while maintaining a minimal computational time. The vibrational Hamiltonian for bound and unbound surfaces is solved within a discrete variable representation (DVR) framework. Properties and energies of excited states are computed using electron correlated singles and doubles equation-of-motion (EOM-CCSD) and similarity transformed equation-of-motion (STEOM-CCSD) methods as implemented in ACESII. The novelty of this procedure is that it is designed to work for medium-sized molecules (size limited by the choice of electronic structure method, not vibrational degrees of freedom) with one or more photodissociation pathways. The theoretical absorption cross-section of NaOH is presented as a small-scale example.