Anharmonicity and quantum effects in vibrational spectroscopy: semiclassical spectroscopy from small molecules to solvated biomolecules

The hallmark of semiclassical dynamics is the ability to get quantum effects starting from classical
trajectories.[1] Therefore, the main challenge semiclassical methods have to face is to demonstrate
their accuracy and possibility to be applied even to large and complex systems.[2] For such systems
classical simulations are widely employed. I will show that while appropriate classical simulations
are able to describe the anharmonicity of the system, a semiclassical treatment can also add
quantum effects.[3] Semiclassical dynamics can be straightforwardly interfaced to different
descriptions of the potential energy surface (PES), ranging from ab initio PESs[4-6] to force
fields[7,8] and QM/MM schemes. This allows one to apply semiclassical spectroscopy to the
calculation of the quantum vibrational frequencies of very different systems, including not only
small molecules characterized by elusive Fermi resonances, like ethanol, or hard-to-assign
experimental spectra, like proline, but also large systems like solvated biomolecules. Finally,
ongoing new efforts to reproduce also the intensity of absorption in the framework of semiclassical
dynamics will be illustrated.