  A time-dependent vibrational electronic coupled-cluster (VECC) approach is
proposed to simulate photoelectron/ UV-VIS absorption spectra, as well as
time-dependent properties for non-adiabatic vibronic models, going beyond the
Born-Oppenheimer approximation. A detailed derivation of the equations of
motion and a motivation of the ansatz are presented. The VECC method employs
second-quantized bosonic construction operators and a mixed linear and
exponential ansatz to form a compact representation of the time-dependent
wave-function. Importantly, the method does not require a basis set, has only
few user-defined inputs, and has a classical (polynomial) scaling with respect
to the number of degrees of freedom (of the vibronic model), resulting in a
favourable computational cost. In benchmark applications to small models and
molecules the VECC method provides accurate results, compared to
Multi-Configurational Time-dependent Hartree (MCTDH) calculations when
predicting short-time dynamical properties (i.e. photo-elecron / UV-VIS
absorption spectra) for non-adiabatic vibronic models. To illustrate the
capabilities the VECC method is also applied successfully to a large vibronic
model for hexahelicene with 14 electronic states and 63 normal modes, developed
in the group by Santoro.
