The Jahn-Teller potential energy surface of the cyclobutane radical cation is
reinvestigated at the CCSD(T)/TZP//RMP2/6-311G(2d,p) level. It is found that
out-of-plane deformations come into play as one moves away from the D4h geometry.
The relaxed radical cation has two potential energy minima, a planar rectangle
1.7 kcal/mol below a puckered rhombus, which are interconnected by a transition
state lying 1.5 kcal/mol above the latter. However, upon inclusion of zero-point
energies, the transition state falls 0.1 kcal/mol below the puckered minimum and 
the enthalpies at 0 K of the three stationary points all lie within 0.54 kcal/mol.
This indicates that the potential energy surface of this cation is strongly
anharmonic. The repercussions of these findings with regards to recent experiments
are discussed.