Validity of pair truncations with effective interaction in Ca isotopes

Using the GXPF1A interaction for the pf shell nuclei, we calculate energy levels and E2 transition rates for the semimagic nuclei Ca43-46 within both the exact shell-model space and a number of subspaces constructed using collective nucleon pairs. We present explicitly the overlaps between wave functions of low-lying states obtained from shell-model calculations and those obtained using truncated nucleon-pair subspaces. These examples are used as touchstones of pair approximations.