  The electronic energy levels of cyclo(Glycine-Phenylalanine),
cyclo(Tryptophan-Tyrosine) and cyclo(Tryptophan-Tryptophan) dipeptides are
investigated with a joint experimental and theoretical approach.
Experimentally, valence photoelectron spectra in the gas phase are measured
using VUV radiation. Theoretically, we first obtain low-energy conformers
through an automated conformer-rotamer ensemble sampling scheme based on
tight-binding simulations. Then, different first principles computational
schemes are considered to simulate the spectra: Hartree-Fock (HF), density
functional theory (DFT) within the B3LYP approximation, the quasi--particle GW
correction, and the quantum-chemistry CCSD method. Theory allows to assign the
main features of the spectra. A discussion on the role of electronic
correlation is provided, by comparing computationally cheaper DFT scheme (and
GW) results with the accurate CCSD method.
