Density functional theory predictions for small radicals containing boron and aluminium: broken symmetry problems and solutions

Selected exchange-correlation functionals were employed to study certain radicals for which unrestricted (U) Hartree-Fock and post-Hartree-Fock methods showed spatial symmetry breakings. For AlO, BO2 and BS2 all functionals produced symmetry adapted solutions and predicted geometries and vibrational spectra in good accord with experiment. The USVWN and UB3LYP solutions for the Dinfinity h O-Al-O structure break spin symmetry. Mixing of these spin-unrestricted Kohn-Sham orbitals yielded stable solutions with reasonable geometries and energetics but with large errors for the vibrational spectra. Only UBHandHLYP results in a broken spatial symmetry solution and yields an anomalous vibrational spectrum. The UBLYP solution does not show a tendency to instability and predicts a Dinfinity h O-Al-O species with a 2€g electronic state.