Wavefunction quality and error estimation of single- and multi-reference coupled-cluster and CI methods: The model system

Different diagnostics for the approximation quality of wavefunction methods are discussed: the weighted T 1 and D 1 norm, the deviation from the full configuration interaction wavefunction, and the variance of the trial wavefunction. All diagnostics are evaluated for the H 4 model system. Various wavefunction based approaches are analyzed. The wavefunction variance performs as a very reliable quantity to estimate the accuracy of the wavefunction and the energy expectation value. Although significantly more expensive than the evaluation of a plain energy expectation value the calculation of the wavefunction variance seems to be a comprehensive substitute for a full configuration interaction calculation.