# Correlation consistent Composite Approach (ccCA)¶

The CCCA module calculates the total energy using the correlation consistent Composite Approach (ccCA). At present the ccCA module is designed for the study of main group species only.

where EMP2/CBS is the complete basis set extrapolation of MP2 energies with the aug-cc-pVnZ (n=T,D,Q) series of basis sets, $\Delta E_{CC}$ is the correlation correction,

$\Delta E_{CV}$ is the core-valence correction,

where $FC1$ symbolizes that all electrons of first-row atoms are correlated, all electrons of second-row atoms are correlated except the 1s MOs, and all electrons of atoms K–Kr are correlated except the 1s2s2p MOs.
$\Delta E_{SR}$ is the scalar-relativistic correction,

and $\Delta E_{ZPE}$ is the zero-point energy correction or thermal correction. Geometry optimization and subsequent frequency analysis are performed with B3LYP/cc-pVTZ.

Suggested reference: N.J. DeYonker, B. R. Wilson, A.W. Pierpont, T.R. Cundari, A.K. Wilson, Mol. Phys. 107, 1107 (2009). Earlier variants for ccCA algorithms can also be found in: N. J. DeYonker, T. R. Cundari, A. K. Wilson, J. Chem. Phys. 124, 114104 (2006).

The ccCA module can be used to calculate the total single point energy for a fixed geometry and the zero-point energy correction is not available in this calculation. Alternatively the geometry optimization by B3LYP/cc-pVTZ is performed before the single point energy evaluation. For open shell molecules, the number of unpaired electrons must be given explicitly.

CCCA

[(ENERGY||OPTIMIZE)   default ENERGY]
[(DFT||DIRECT)   default DFT]
[(MP2||MBPT2)   default MP2]
[(RHF||ROHF||UHF)   default RHF]
[(CCSD(T)||TCE)   default CCSD(T)]
[NOPEN   <integer number of unpaired electrons   default   0 >]
[(THERM||NOTHERM)   default  THERM]
[(PRINT||NOPRINT) default NOPRINT]
[BASIS <basis name for orbital projection guess>]
[MOVEC <file name for orbital projection guess>]
END


One example of input file for single point energy evaluation is given here:

start h2o_ccca

title "H2O, ccCA test"

geometry units au

H       0.0000000000   1.4140780900  -1.1031626600
H       0.0000000000  -1.4140780900  -1.1031626600
O       0.0000000000   0.0000000000  -0.0080100000
end



An input file for the ground state of O2 with geometry optimization is given below:

start o2_ccca

title "O2, ccCA test"

geometry units au

O       0.0000000000   0.0000000000  -2.0000
O       0.0000000000   0.0000000000   0.0000

end

ccca
optimize
dft
nopen 2
end