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BeC2 crystallizes in the tetragonal P4_2/mnm space group. The structure is one-dimensional and consists of two BeC2 ribbons oriented in the (1, 1, 0) direction. Be2+ is bonded in a linear geometry to two equivalent C1- atoms. Both Be–C bond lengths are 1.75 Å. C1- is bonded in a distorted linear geometry to one Be2+ and one C1- atom. The C–C bond length is 1.26 Å.
The coupled-cluster and Brueckner electron correlation methods are used to determine the equilibrium structures, dipole moments, harmonic vibrational frequencies, and IR intensities of NH3, FON, Be3, BeC2, and BeO2. The singles and doubles coupled-cluster and Brueckner doubles methods are employed, and the corresponding methods that include a perturbational estimate of connected triple excitations are also investigated. The T sub 1 diagnostic is found to provide a good indication of the magnitude of the difference between the results obtained with the coupled-cluster and Brueckner methods. For NH3, the T sub 1 diagnostic is small and so the differences between results obtained from coupled-cluster and Brueckner theories are quite small. It is found for all of the molecules under consideration that inclusion of the contribution from connected triple excitations is more important than the differences between the Brueckner and coupled-cluster correlation methods.