title: Calculation of absorption spectra involving multiple excited states: approximate methods based on the mixed quantum classical Liouville equation.
abstract: We investigate the calculation of absorption spectra based on the mixed quantum classical Liouville equation (MQCL) methods. It has been shown previously that, for a single excited state, the averaged classical dynamics approach to calculate the linear and nonlinear spectroscopy can be derived using the MQCL formalism. This work focuses on problems involving multiple coupled excited state surfaces, such as in molecular aggregates and in the cases of coupled electronic states. A new equation of motion to calculate the dipole-dipole correlation functions within the MQCL formalism is first presented. Two approximate methods are then proposed to solve the resulted equations of motion. The first approximation results in a mean field approach, where the nuclear dynamics is governed by averaged forces depending on the instantaneous electronic states. A modification to the mean field approach based on first order moment expansion is also proposed. Numerical examples including calculation of the absorption spectra of Frenkel exciton models of molecular aggregates, and the pyrazine molecule are presented.
authors.affiliation: Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100190, China.
authors.forename: Shuming
authors.initials: S
authors.lastname: Bai
authors.affiliation: Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100190, China.
authors.forename: Weiwei
authors.initials: W
authors.lastname: Xie
authors.affiliation: Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100190, China.
authors.forename: Lili
authors.initials: L
authors.lastname: Zhu
authors.affiliation: Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun, Beijing 100190, China.
authors.forename: Qiang
authors.initials: Q
authors.lastname: Shi