The D-5h -> D-10h isomerization in the C-10 carbon cluster is investigated at the relativistic, all-electron CCSDT(Q)/CBS level. Previous high-level studies examined this isomerization at the valence CCSD(T)/CBS level. We show that capturing this isomerization energy requires accurate treatment of the CCSD(T)/CBS, post-CCSD(T), core-valence, scalar relativistic, diagonal Born-Oppenheimer, and zero-point vibrational energy components. Combining these components shows that the two structures are practically isoenergetic at 0 K (i.e., the D-5h structure is more stable by merely + 0.100 kcal mol(-1)). We also show that computationally economical composite protocols erroneously predict that the D-10h structure is energetically more stable at 0 K. (C) 2018 Elsevier B.V. All rights reserved.