A Nonperturbative Methodology for Simulating Multidimensional Spectra of Multiexcitonic Molecular Systems via Quasiclassical Mapping Hamiltonian Methods.

We present a new methodology for simulating multidimensional electronic spectra of complex multiexcitonic molecular systems within the framework of quasiclassical mapping Hamiltonian (QC/MH) methods. The methodology is meant to be cost-effective for molecular systems with a large number of nuclear degrees of freedom undergoing nonequilibrium nonadiabatic dynamics on multiple coupled anharmonic electronic potential energy surfaces, for which quantum-mechanically exact methods are not feasible. The methodology is based on a nonperturbative approach to field-matter interaction, which mimics the experimental measurement of those nonlinear time-resolved spectra via phase cycling and can accommodate laser pulses of arbitrary shape and intensity. The ability of different QC/MH methods to accurately simulate two-dimensional and pump-probe electronic spectra within the proposed methodology is compared in the context of a biexcitonic benchmark model that includes both the singly excited and doubly excited electronic states. The QC/MH methods compared include five variations of the linearized semiclassical (LSC) method and the mean-field (Ehrenfest) method. The results show that LSC-based methods are significantly more accurate than the mean-field method and can yield quantitatively accurate two-dimensional and pump-probe spectra when nuclear degrees of freedom can be treated as classical-like.