Correlation Consistent Composite Approach (ccCA)
This approach, developed at the University of North Texas by Angela K. Wilson's research group, utilizes the correlation consistent basis sets developed by Dunning and co-workers. Unlike the Gaussian-n methods, ccCA does not contain any empirically fitted term. The B3LYP density functional method with the cc-pVTZ basis set, and cc-pV(T+d)Z for third row elements (Na - Ar), are used to determine the equilibrium geometry. Single point calculations are then used to find the reference energy and additional contributions to the energy. The total ccCA energy for main group is calculated by:
- EccCA = EMP2/CBS + ΔECC + ΔECV + ΔESR + ΔEZPE + ΔESO
The reference energy EMP2/CBS is the MP2/aug-cc-pVnZ (where n=D,T,Q) energies extrapolated at the complete basis set limit by the Peterson mixed gaussian exponential extrapolation scheme. CCSD(T)/cc-pVTZ is used to account for correlation beyond the MP2 theory:
- ΔECC = ECCSD(T)/cc-pVTZ - EMP2/cc-pVTZ
Core-core and core-valence interactions are accounted for using MP2(FC1)/aug-cc-pCVTZ:
- ΔECV= EMP2(FC1)/aug-cc-pCVTZ - EMP2/aug-cc-pVTZ
Scalar relativistic effects are also taken into account with a one-particle Douglass Kroll Hess Hamiltonian and recontracted basis sets:
- ΔESR = EMP2-DK/cc-pVTZ-DK - EMP2/cc-pVTZ
The last two terms are Zero Point Energy corrections scaled with a factor of 0.989 to account for deficiencies in the harmonic approximation and spin-orbit corrections considered only for atoms.
Read more about this topic: Quantum Chemistry Composite Methods
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