Accurate harmonic/anharmonic vibrational frequencies for open-shell systems: performance of the B3LYP/N07D model for semirigid free radicals benchmarked by CCSD(T) computations

Impressive growth of computer facilities and effective implementation of very accurate quantum mechanical methods allow, nowadays, the determination of structures and vibrational characteristics for small- to medium-sized molecules to a very high accuracy. Since the situation is much less clear for open-shell species, we decided to build a suitable database of harmonic and anharmonic frequencies for small-sized free radicals containing atoms of the first two rows of the periodic table. The level of theory employed is the CCSD(T) model in conjunction with triple- and quadruple-zeta basis sets, whose accuracy has been checked with respect to the available experimental data and/or converged quantum mechanical computations. Next, in view of studies of larger open-shell systems, we have validated the B3LYP/N07D model with reference to the above database: our results confirm previous suggestions about the remarkable reliability and reduced computational cost of this computational method. A number of test computations show that basis set extension has negligible effects and other density functionals (including last generation ones) deliver significantly worse results. Increased accuracy can be obtained, instead, by using CCSD(T) harmonic frequencies and B3LYP/N07D anharmonic corrections.