wB97Xd/Def2-SVP, DCM, 

HCCH                         HCC• + H•                        CC + H•

-77.211099  =   (-76.499229  +  -0.499536)  = (-75.768761  +  -0.499536)

-77.211099  = -76.998765, -76.499229  =  -76.268297
      
ΔH  133.2                   ΔH     144.9 

ΔΔH  = +11.7

wB97Xd/Def2-SVP, DCM, 

HCCH                         HCC• + H•                        CC + H•

-77.2421636  =   (-76.5181141317  +  -0.5018966)  = (-75.7764210968  +  -0.5018966)

-77.2421636  = -77.0200107317, -76.5181141317  =  -76.2783176968
      
ΔE  139.4                   ΔE     150.4

ΔΔE  = +11.0

CCSD(T)/Def2-TZVPPD, DCM, Total CCSD(T) Energies

HCCH                         HCC• + H•                        CC + H•

-77.1930755  =   (-76.4720465  +  -0.4998248)  = (-75.7863285  +  -0.4998248)

-77.1930755  = -76.9718713,  -76.4720465  =  -76.2861533
      
ΔE  138.8                   ΔE     116.6 
      
ΔΔE = -22.2 kcal/mol

CCSD(T)/Def2-TZVPPD , DCM, Thermal enthalpy corrected CCSD(T) Energies

HCCH                         HCC• + H•                        CC + H•

-77.162851  =   (-76.453745  +  -0.497464)  = (-75.778817  +  -0.497464)

-77.162851  = -76.951209,  -76.453745  =  -76.276281
      
ΔH  132.8                   ΔH     111.4 
      
ΔΔH = -21.4 kcal/mol


CCSD(T)/Def2-TZVPPD,  Gas phase

HCCH                         HCC• + H•                        CC + H•

-77.1901039  =   (-76.4699305  +  -0.4998248)  = (-75.7853769  +  -0.4998248)

-77.1901039   = -76.9697553,  -76.4699305  =  -76.2852017
      
ΔE  138.3                   ΔE     115.9
      
ΔΔE = -22.3 kcal/mol


Table 2 in D. Danovich, P. C. Hiberty, W. Wu, H. S. Rzepa and S. Shaik, 
"The Nature of the 4th Bond in the Ground State of C2: The Quadruple Bond Conundrum" 
Chem. Euro. J., 2014, 20, 6220–6232. DOI: https://doi.org/10.1002/chem.201400356

ΔΔE(H) 15.0 - 16.8 kcal/mol (exp).  CCSD(T)/Def2-TZVPPD needs correction by ~+4.6 kcal/mol
for the excessive calculated stability of C2 predicted by this method.wB97Xd/Def2-SVP needs correction by
-27.8