Atomization energies of the carbon clusters Cn (n=2-10) revisited by means of W4 theory as well as density functional, Gn, and CBS methods

The thermochemistry of the carbon clusters C-n (n = 2 - 10) has been revisited by means of W4 theory and W3.2lite theory. Particularly, the larger clusters exhibit very pronounced post-CCSD(T) correlation effects. Despite this, our best calculated total atomization energies agree surprisingly well with the 1991 estimates obtained from scaled CCD(ST)/6-31G* data. Accurately reproducing the small singlet-triplet splitting in C-2 requires inclusion of connected quintuple and sextuple excitations. Post-CCSD(T) correlation effects in C-4 stabilize the linear form. Linear/cyclic equilibria in C-6, C-8, and C-10 are not strongly affected by connected quadruples, but they are affected by higher-order triples, which favor polyacetylenic rings but disfavor cumulenic rings. Near the CCSD(T) basis set limit, C-10 does undergo bond angle alternation in the bottom-of- the-well structure, although it is expected to be absent in the vibrationally averaged structure. The thermochemistry of these systems, and particularly the longer linear chains, is a particularly difficult test for density functional methods. Particularly for the smaller chains and the rings, double-hybrid functionals clearly outperform conventional DFT functionals for these systems. Among compound thermochemistry schemes, G4 clearly outperforms the other members of the Gn family. Our best estimates for total atomization energies at 0K are: C-2((1)Sigma(+)(g)) 144.07, C-2((3)Pi(u)) 142.39, C-3((1)Sigma(+)(g)) 315.83, C-4((3)Sigma(-)(g)) 429.16, C-4((1)A(g)) 430.09, C-5((1)Sigma(+)(g)) 596.64, C-6((3)Sigma(-)(g)) 717.19, C-6((1)A(1)') 729.68, C-7((1)Sigma(+)(g)) 877.45, C-8(Sigma(-)(g)) 1001.86, C-8((1)A(g)) 1014.97, C-9((1)Sigma(+)(g)) 1159.21, C-10((3)Sigma(-)(g)) 1288.22, and C-10((1)A(1)') 1355.54 kcal mol(-1).