“…The AM1 semiempirical model Hamiltonian fails to describe the cyclobutane ring puckering, giving a planar form. However, by experiment 4, 8–10, 13, 15 and by B3LYP/6‐311++G(3df,pd), it is folded diagonally across the four‐membered ring with a CCCC torsional angle by B3LYP of 17.69°; although the difference of energy between both forms, planar (saddle) and nonplanar, is very small, 0.869 kcal/mol by B3LYP/6‐311++G(3df,pd). Analogously, AM1 fails in azetidine with a NCCC torsional angle of 2.41° (α = 3.47°) when by experiment 13, by HF/6‐31G** with 17.53° (α = 25.47°), by MP2/6‐31G** with 22.29° (α = 32.20°), and by B3LYP/6‐311++G(3df,pd) with 17.94° (α = 25.97°) is nonplanar.…”