Knowledge Bank

Equilibrium structures, binding energies and harmonic vibrational frequencies of the $Be3,Be4,Mg3$ and $Mg4$ clusters have been determined using large Gaussian basis sets in conjunction with highly correlated wave functions. The electron correlation methods used including singles and doubles coupled-cluster [CCSD] theory and the extension including the effects of connected triple excitations [CCSD(T)] along with the multireference configuration interaction [MRCI] approach. In addition, full cubic and quartic force fields have been determined (with the CCSD(T) method) and used in evaluating anharmonic constants, vibration-rotation interaction constants and quartic and sextic centrifugal distortion constants. Infrared absorption intensities have been determined using the double harmonic approximation. The anharmonic analyses display some very interesting features which will be discussed.