Our BeBe distances turn out to be very slightly smaller (by about
0.01 Å) than the already small values found by Zhao et
al. .[13] We notice that the BeBe
separations in the D 3h[NgBeH3BeNg]+ cations areca . 0.04 Å shorter than those found for the
rhombic Be2O2
cluster.[8-9, 11] We are not aware of any
previously reported geometries for the magnesium systems. We find that
the MgMg separation in the D 3h[HeMgH3MgHe]+ cation is shorter byca . 0.03 Å than was found for the rhombic
Mg2O2 cluster.[9,
11]
The total energies from calculations at various levels of theory are
reported in Table S1 in the Supporting Information. For each system, the
energy difference between the SCGVB(6) and the corresponding CASSCF(6,6)
descriptions was very small (ca . 3 kJ/mol); the SCGVB(6)
wavefunctions for the various Be and Mg systems incorporate more than
96.8% and 99.4%, respectively, of the non-dynamical correlation energy
that is retrieved with the CASSCF(6,6) calculations. Increasing the
number of active orbitals to 11, as in CASSCF(6,11), retrieved further
energy lowerings in each case of ca . 120 kJ/mol. Much larger
energy improvements are of course associated with the dynamical
correlation incorporated in the all-electron CCSD(T) calculations. It
remains to be seen, however, whether our various modes of analysis
reveal any significant differences in the nature of the bonding when we
switch to different levels of theory.