Multimode quantum dynamics with multiple Davydov D₂ trial states: Application to a 24-dimensional conical intersection model

The ultrafast nonadiabatic dynamics of a two-electronic-state four-vibrational-mode conical intersection coupled to a finite bath with up to 20 harmonic oscillators has been investigated by employing the multiple Davydov D₂ ansatz. It is demonstrated, using the multi-configuration time-dependent Hartree method as a benchmark, that this approach provides an efficient and robust description of the internal conversion process at multimode conical intersections. Thanks to the Gaussian nature of the Davydov ansatz, it allows for numerically accurate simulations of time-dependent diabatic and (for the first time for a 24-mode system) adiabatic populations of the electronic states and reduced probability densities of the tuning and coupling modes. The obtained adiabatic populations and wave packets can be used as benchmarks for the testing of various simulation methods, in particular, surface-hopping methods.