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Goal: select the most accurate calculation that is computationally feasible for a given molecular system
Model chemistry: theoretical method and basis set
HF MP2 CCSD CCSD(T) CCSDT … Full CI
Minimal …Split-
valence …
Polarized …
Diffuse …High
angular momentum
…
… … … … … … … …
∞ … Exact solution
• size-consistent – a calculation that gives the same energy for two atoms (or molecular fragments) separated by a large distance as is obtained from summing the energies for the atoms (or molecular fragments) computed separately. So for a size-consistent method, the bond energy in N2 is ΔEdiss = 2E(N) - E(N2). For a method that is not size-consistent, a "supermolecule" calculation with a big distance (e.g., 100 Å) is required: ΔEdiss = E(N......N) - E(N2).
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