  We have performed quantum Monte Carlo calculations to study the cohesion
energetics of carbon allotropes, including $sp^3$-bonded diamond, $sp^2$-bonded
graphene, $sp$-$sp^2$ hybridized graphynes, and $sp$-bonded carbyne. The
computed cohesive energies of diamond and graphene are found to be in excellent
agreement with the corresponding values determined experimentally for diamond
and graphite, respectively, when the zero-point energies, along with the
interlayer binding in the case of graphite, are included. We have also found
that the cohesive energy of graphyne decreases systematically as the ratio of
$sp$-bonded carbon atoms increases. The cohesive energy of $\gamma$-graphyne,
the most energetically-stable graphyne, turns out to be 6.766(6) eV/atom, which
is smaller than that of graphene by 0.698(12) eV/atom. Experimental difficulty
in synthesizing graphynes could be explained by their significantly smaller
cohesive energies. Finally we conclude that the cohesive energy of a
newly-proposed graphyne can be accurately estimated with the carbon-carbon bond
energies determined from the cohesive energies of graphene and three different
graphynes considered here.
