  The accurate determination of the preferred ${\rm Si}_{12}{\rm C}_{12}$
isomer is important to guide experimental efforts directed towards synthesizing
SiC nano-wires and related polymer structures which are anticipated to be
highly efficient exciton materials for opto-electronic devices. In order to
definitively identify preferred isomeric structures for silicon carbon
nano-clusters, highly accurate geometries, energies and harmonic zero point
energies have been computed using coupled-cluster theory with systematic
extrapolation to the complete basis limit for set of silicon carbon clusters
ranging in size from SiC$_3$ to ${\rm Si}_{12}{\rm C}_{12}$. It is found that
post-MBPT(2) correlation energy plays a significant role in obtaining converged
relative isomer energies, suggesting that predictions using low rung density
functional methods will not have adequate accuracy. Utilizing the best
composite coupled-cluster energy that is still computationally feasible,
entailing a 3-4 SCF and CCSD extrapolation with triple-$\zeta$ (T) correlation,
the {\it closo} ${\rm Si}_{12}{\rm C}_{12}$ isomer is identified to be the
preferred isomer in support of previous calculations [J. Chem. Phys. 2015, 142,
034303]. Additionally we have investigated more pragmatic approaches to
obtaining accurate silicon carbide isomer energies, including the use of frozen
natural orbital coupled-cluster theory and several rungs of standard and
double-hybrid density functional theory. Frozen natural orbitals as a way to
compute post MBPT(2) correlation energy is found to be an excellent balance
between efficiency and accuracy.
