Geometry, vibrational frequencies, and ionization energies of BeX2 (X = F,Cl, Br, and I)

The equilibrium geometries and harmonic vibrational frequencies of BeF2, Be Cl2, BeBr2, and BeI2 are calculated. For the lighter two molecules, only al l-electron methods were used, whereas for the heavier two, all-electron and effective core potential (ECP) methods were used. MP2. QCISD, and B3LYP ca lculations were employed, and it was found that the combination of ECPs (wi th a suitable valence basis set) with the B3LYP density functional method i s a relatively inexpensive means of obtaining reliable information on these species. All four molecules were found to be linear, in agreement with sim ple molecular orbital ideas. The lowest electronic states are calculated us ing the configuration interaction with single excitations approach. Finally the adiabatic and vertical ionization energies of the BeX2 species are cal culated. For BeF2 and BeCl2, the first vertical ionization energy (VIE) was calculated at the CASSCF+MRCI level, and the lowest four VIEs were calcula ted at the CCSD(T) level using large basis sets: for BeCl2 comparison with the photoelectron spectrum is presented. For BeBr2 and BeI2, the lowest VIE s were calculated using a density functional approach, including spinorbit effects. During the calculation of the adiabatic ionization energies, the e quilibrium geometries of the BeX2+ species were calculated: it was found th at BeF2+ and BeCl2+ had bent equilibrium geometries (corresponding to the ( X) over tilde(2)B(2) Renner-Teller component of the lowest (2)Pi(g) state, in a linear configuration), whereas BeBr2+ and BeI2+ have linear (X(2)Pi g, ) equilibrium geometries. The calculated adiabatic ionization energies for BeF2 and BeCl2 are significantly higher than previously accepted values.