  The energy positions of frontier orbitals in organic electronic materials are
often studied experimentally by (inverse) photoemission spectroscopy and
theoretically within density functional theory. However, standard
exchange-correlation functionals often result in too small fundamental gaps,
may lead to wrong orbital energy ordering, and do not capture
polarization-induced gap renormalization. Here, we examine these issues and a
strategy for overcoming them by studying the gas phase and bulk electronic
structure of the organic molecule quinacridone (5Q), a promising material with
many interesting properties for organic devices. Experimentally, we perform
angle-resolved photoemission spectroscopy (ARUPS) on thin films of the
crystalline $\beta$-phase of 5Q. Theoretically, we employ an optimally-tuned
range-separated hybrid functional (OT-RSH) within density functional theory.
For the gas phase molecule, our OT-RSH result for the ionization potential (IP)
represents a substantial improvement over the semi-local PBE and the PBE0
hybrid functional results, producing an IP in quantitative agreement with
experiment. For the bulk crystal, we take into account the correct screening in
the bulk, using the recently developed OT-SRSH approach, while retaining the
optimally-tuned parameters for the range-separation and the short-range Fock
exchange. This leads to a band gap narrowing due to polarization effects and
results in a valence band spectrum in excellent agreement with experimental
ARUPS data, with respect to both peak positions and heights. Finally,
full-frequency $G_0W_0$ results based on a hybrid functional starting point are
shown to agree with the OT-SRSH approach, improving substantially on the
PBE-starting point.
