Cyanovinyl radical: an illustration of the poor performance of unrestricted perturbation theory and density functional theory procedures in calculating radical stabilization energies

Stabilization energies for the 1-cyanovinyl radical (CH2=(CCN)-C-.) have be en calculated using a variety of conventional ab initio (Moller-Plesset, qu adratic configuration interaction and coupled-cluster) and density function al theory (B-LYP, B3-LYP) procedures, as well as with a range of compound m ethods. Compared with a high-level benchmark value (that predicts a stabili zation energy of 17.1 kJ mol(-1)), UMP2 and UMP4 give the wrong sign and ma gnitude of the stabilization energy (both methods predicting destabilizatio n instead of stabilization), while B-LYP and B3-LYP overestimate the degree of stabilization. The RMP2, RMP4, QCISD(T) and CCSD(T) techniques, and sev eral, but not all, variants of G2 and CBS theories give radical stabilizati on energies in good agreement with the benchmark value.