  We describe the implementation of orbital optimization for the models in the
perfect pairing hierarchy [Lehtola et al, J. Chem. Phys. 145, 134110 (2016)].
Orbital optimization, which is generally necessary to obtain reliable results,
is pursued at perfect pairing (PP) and perfect quadruples (PQ) levels of theory
for applications on linear polyacenes, which are believed to exhibit strong
correlation in the {\pi} space. While local minima and {\sigma}-{\pi} symmetry
breaking solutions were found for PP orbitals, no such problems were
encountered for PQ orbitals. The PQ orbitals are used for single-point
calculations at PP, PQ and perfect hextuples (PH) levels of theory, both only
in the {\pi} subspace, as well as in the full {\sigma}{\pi} valence space. It
is numerically demonstrated that the inclusion of single excitations is
necessary also when optimized orbitals are used. PH is found to yield good
agreement with previously published density matrix renormalization group (DMRG)
data in the {\pi} space, capturing over 95% of the correlation energy.
Full-valence calculations made possible by our novel, efficient code reveal
that strong correlations are weaker when larger bases or active spaces are
employed than in previous calculations. The largest full-valence PH
calculations presented correspond to a (192e,192o) problem.
