Geometries and vibrational frequencies of small radicals: Performance of Coupled Cluster and more approximate methods

We generated a new set of reference geometries of small radicals using experimental equilibrium structures, as well as a benchmark-quality coupled cluster additivity scheme including up to quadruples excitations (CCSDTQ). Using these geometries and a set of experimental vibrational frequencies of open shell diatomics, we evaluated the performance of various coupled cluster methods based mainly on unrestricted references, using Dunning basis sets both with and without core correlation. Contrary to previous results, we found that UCCSD(T) and ROCCSD(T) perform equally well for geometries, better than CCSD, and close to their performances for closed shell systems. No improvement over CCSD(T) was achieved by using a Brueckner reference (BD(T)) or full triples (CCSDT). For frequencies, ROCCSD(T), BD(T), and CCSDT improve upon UCCSD(T), especially for the troublesome NO and CO+ cases. EOMIP-CCSD yields geometries and harmonic frequencies similar to CCSD, and qualitatively correct anharmonic (VPT2) contributions in all cases, like the RO-CC methods. The double hybrid DFT functional B2PLYP-D yields geometries and frequencies of similar quality to that of CCSD but at a much reduced cost. The meta hybrid functionals M06-2X, M06-HF, and BMK perform worse than CCSD, and worse than B3LYP, on average.