Summary

Preliminary results obtained with calculations on the Rh₈ cation at the BLYP/LANL2DZ level of theory. Relative energies show the cube structure to be the putative global minimum (FIGURE 2). Although the non-cube structures are all descreasing in relative energy as the spin multiplicity increases, higher multiplicities do not get close to the cube energy in the octet state (checked up to 14-tet). Equilibrium abundances were calculated in the microcanonical ensemble (following the procedure in JCP, vol 124, p. 204317, (2006)) for a range of total energies and for the octet structures only(FIGURE 3). The cube dominates the abundance at low energies - at higher energies the bicapped trigonal prism and the square antiprism take over. On this basis, I calculated the IR spectra (using a broadening of 10 cm-1) for the cube, bicapped trigonal prism and the square antiprism octets (FIGURE 4). All show a peak in the region between 200 and 250 cm-1. Extra peaks are seen for the bicapped trigonal prism - but if the experiments have uncertainty in the peak intensity, it's plausible that they might not appear? To summarise: if these clusters are cold in your experiments, you'll probably see the cube - if not, you could be seeing a mix of bicapped trigonal prism and the square antiprism (?)

To take this further, it might be a good idea to run the abundance calculations over all spin states, and presumably calculate IR spectra for other spin states of the relevant structures.

FIGURE 1: Structures
FIGURE 2: Relative energies
FIGURE 3: Equilibrium abundances (octet structures)
FIGURE 4: IR Spectra (octet structures)

Structures

1. Cube	2. Bicapped octahedron	3. Diamond prism	4. Bicapped trigonal prism
5. Distorted capped pentagonal bipyramid	6. Square antiprism	7. Capped pentagonal bipyramid	8. Bicapped octahedron 2
9. Bicapped octahedron 3	10. Bicapped boat

Energies

Equilibrium Abundances

Infrared spectra