  Unitary cluster expansions of the electronic wavefunction have recently
gained much interest because of their use in conjunction with quantum
algorithms. In this contribution, we investigate some aspects of an ansatz
using generalized two-body excitations operators, which has been considered in
some recent works on quantum algorithms for quantum chemistry. Our numerical
results show that in particular two-body operators with effective particle-hole
excitation level of one in connection with the usual particle-hole double
excitation operators lead to a very accurate yet compact representation of the
wavefunction. Generalized two-body operators with effective excitation rank
zero have a considerably less pronounced effect. We compare to standard and
unitary coupled-cluster expansions and show that the above mentioned approach
matches or even surpasses the accuracy of expansions with three-body
particle-hole excitations, in particular at the onset of strong correlation. A
downside of the approach is that it is rather difficult to rigorously converge
it to its variational minimum.
