  The simple ultraviolet absorption spectrum of thiophene is investigated using
a combination of a vibronic coupling model Hamiltonian with multi-configuration
time-dependent Hartree quantum dynamics simulations. The model includes five
states and all twenty-one vibrations, with potential surfaces calculated at the
complete active space with second-order perturbation (CASPT2) level of theory.
The model includes terms up to seventh-order to describe the diabatic
potentials. The resulting spectrum is in excellent agreement to the
experimentally measured spectrum of Holland et al [PCCP (2014) 16: 21629]. The,
until now not understood, spectral features are assigned, with a combination of
strongly coupled vibrations and vibronic coupling between the states giving
rise to a progression of triplets on the rising edge of the broad spectrum. The
analysis of the underlying dynamics indicates that population transfer between
all states takes place on a sub-100 fs timescale, with ring-opening occurring
at longer times. The model thus provides a starting point for further
investigations into the complicated photo-excited dynamics of this key
hetero-aromatic molecule.
