As said earlier, the phenyl radical is the only open-shell species in the B2se set. That is why it was excluded from the GEO statistics given in Section 3, and we analyse it separately here. Figure tween these two plots is the position of MP2, which was one of the top performers for benzene, whereas it stands out as by far the worst for the phenyl radical. The MP2 calculation here is based on the unrestricted HF (UHF), which is severely spin-contaminated ( S 2 off by more than 50% for the considered radical), and in such cases MP2 typically gives very bad geometries. Although spin-contaminated, UHF itself gives way more reasonable geometry for the phenyl radical than MP2 [Figure ]. The inset of Figure (a) explores GEO values for different methods built upon MP2. It is of no surprise 42 that GEO of SCS-MP2 40 is just slightly lower than that of MP2, as SCS-MP2 also uses the UHF orbitals here. On the other hand, B2PLYP and the two orbital-optimized (OO) MP2 approaches (OO-MP2 and OO-SCS-MP2 ) reduce spin contamination of MP2, and thus give highly accurate structures for the phenyl radical (GEO values ∼0.05 kcal/mol). One should also note that B2PLYP is the cheapest of the three methods as its cost is about the same as MP2, while orbital optimizations make the two OO-MP2 approaches substantially more expensive than MP2.