The positive semidefinite (psd) rank of a polytope is
the size of
the smallest psd cone that admits an affine slice that projects linearly onto
the polytope. The psd rank of a
d -polytope is at least
class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0097316516300747&_mathId=si1.gif&_user=111111111&_pii=S0097316516300747&_rdoc=1&_issn=00973165&md5=d9eb65dbff49e37ede1f981861e424c6" title="Click to view the MathML source">d+1class="mathContainer hidden">class="mathCode">, and when equality holds we say that
the polytope is psd-minimal. In this paper we develop new tools for
the study of psd-minimality and use
them to give a complete
classification of psd-minimal 4-polytopes. The main tools introd
uced are trinomial obstr
uctions, a new algebraic obstr
uction for psd-minimality, and
the slack ideal of a polytope, which encodes
the space of realizations of a polytope up to projective equivalence.
Our central result is that there are 31 combinatorial classes of psd-minimal 4-polytopes. We provide combinatorial information and an explicit psd-minimal realization in each class. For 11 of these classes, every polytope in them is psd-minimal, and these are precisely the combinatorial classes of the known projectively unique 4-polytopes. We give a complete characterization of psd-minimality in the remaining classes, encountering in the process counterexamples to some open conjectures.