The structures, binding energies and vibrational frequencies of Ca₃ and Ca₄ - An application of the CCSD(T) method

The Ca₃ and Ca₄ metallic clusters have been investigated using state-of-the-art ab initio quantum mechanical methods. Large atomic natural orbital basis sets have been used in conjunction with the singles and doubles coupled-cluster (CCSD) method, a coupled-cluster method that includes a perturbational estimate of connected triple excitations, denoted CCSD(T), and the multireference configuration interaction (MRCI) method. The equilibrium geometries, binding energies and harmonic vibrational frequencies have been determined with each of the methods so that the accuracy of the coupled-cluster methods may be assessed. Since the CCSD(T) method reproduces the MRCI results very well, cubic and quartic force fields of Ca₃ and Ca₄ have been determined using this approach and used to evaluate the fundamental vibrational frequencies. The infrared intensities of both the e′ mode of Ca₃ and the t₂ mode of Ca₄ are found to be small. The results obtained in this study are compared and contrasted with those from our earlier studies on small Be and Mg clusters.