Initial applications of an exponential multi-reference wavefunction ansatz

Initial applications of a new exponential multi-reference wavefunction ansatz M. Hanrath, J. Chem. Phys. 123 (2005) 084102] are reported. The proposed ansatz is based on the state universal ansatz of Jeziorski and Monkhorst B. Jeziorski, H.J. Monkhorst, Phys. Rev. A 24 (1981) 1668] and uses a reference specific cluster operator with a determinant based indexing. The approach is Fermi vacuum invariant, size consistent, insensitive with respect to intruder states and converges to full CI with an increasing cluster operator excitation level. Applications on the H₄ and H₈ model systems are reported.