Performance of the hybrid density functionals in the determination of the geometric structure, vibrational frequency and singlet-triplet energy separation of CH₂, CHF, CF₂, CCl₂ and CBr₂

The geometric structure, vibrational frequency, dipole moment and singlet-triplet energy separation of CH₂, CHF, CF₂, CCl₂ and CBr₂ has been investigated using B3LYP, B3P86 and B3PW91 hybrid density functionals. For CH₂, the density functionals predict the equilibrium C-H bond length, H- C-H bond angle and vibrational frequency to an accuracy comparable to high level ab initio methods. However, only B3LYP functional produces reasonable singlet-triplet energy separation. Also estimates of the singlet-triplet energy separation of CHF and CF₂ indicate B3LYP produces a better agreement with experiment than B3P86 or B3PW91.