Vibrational frequencies of diatomic molecules from Car and Parrinello molecular dynamics

The vibrational frequency of selected diatomic molecules (H₂, N₂, F₂, HF, CO) has been calculated from Car-Parrinello molecular dynamics. The associated electronic energies have been obtained within the Born-Oppenheimer approximation by using the local density functional scheme where plane waves and pseudopotentials are used. Our vibrational frequency results, extrapolated to zero effective fictitious mass values, are in good agreement with the experimental values for H₂, N₂, HF and CO, whereas for F₂ our calculated value is larger. The optimized bond lengths and the harmonic vibrational frequencies obtained by a local density functional scheme with plane waves have been compared with other theoretical methods (Hartree-Fock, MP2, CCSD(T), DFT using Gaussian basis sets). There is good agreement between the LDA procedure using plane-wave or Gaussian-type orbital basis sets.