  In this work we set up a model Hamiltonian to study the excited state quantum
dynamics of 1,1-difluoroethylene, a molecule that has equivalent atoms
exchanged by a torsional symmetry operation leading to equivalent minima on the
potential energy surface. In systems with many degrees of freedom where the
minimum energy geometry is not unique, the ground state wavefunction will be
delocalised among multiple minima. In this small test system, we probe the
excited state dynamics considering localised (in a single minimum) and
delocalised (spread over among multiple minima) wavefunctions and check whether
this choice would influence the final outcome of the quantum dynamics
calculations. Our molecular Hamiltonian comprises seven electronic states,
including valence and Rydberg states, computed with the MS-CASPT2 method and
projected onto the vibrational coordinates of the twelve normal modes of
1,1-difluoroethylene in its vibrational ground state. This Hamiltonian has been
symmetrised along the torsional degree of freedom to make both minima
completely equivalent and the model is supported by the excellent agreement
with the experimental absorption spectrum. Quantum dynamics results show that
the different initial conditions studied do not appreciably affect the excited
state populations or the absorption spectrum when the dynamics is simulated
assuming a delta pulse excitation.
