VIBRATIONAL FREQUENCIES FOR BE-3 AND BE-4

The harmonic vibrational frequencies of the Be3 and Be 4 clusters have been determined using ab initio electronic structure calculations. Large atomic natural orbital (ANO) basis sets have been used in conjunction with high levels of correlation treatment. These include multireference configuration-interaction (MRCI) and singles and doubles coupled-cluster (CCSD) methods, and the CCSD method augmented with a correction for connected triple excitations [CCSD(T)]. In general, all three treatments agree very well. The only substantial disagreement is for the totally symmetric stretching mode in Be3, where the CCSD method yields a harmonic frequency that is 57 cm-1 smaller than the MRCI value. The fundamental vibrational frequencies of Be3 and Be4 have been determined using second-order perturbation theory to obtain anharmonic corrections; Be3 is treated as a symmetric top and Be4 as a spherical top. Full CCSD(T) quartic force fields were used to determine anharmonic constants, vibration-rotation interaction constants, and quartic and sextic centrifugal distortion constants. The anharmonic corrections for the two vibrational modes of Be3 reduce the frequencies by less than 5%, which is typical for bond-stretching vibrations. The a1 and e vibrations of Be4 exhibit somewhat smaller anharmonic corrections that decrease the frequency by about 3%. However, the only IR active mode of Be4 [ω3(t2)] displays a large positive anharmonic correction of + 111 cm-1, or almost 20%. Finally, IR intensities have been determined using the double harmonic approximation. © 1990 American Institute of Physics.