1. Correlated ab Initio Methods

2. Goal: select the most accurate calculation that is computationally feasible for a given molecular system Model chemistry: theoretical method and basis set HFMP2CCSDCCSD(T)CCSDT … Full CI Minimal … Split- valence … Polarized … Diffuse … High angular momentum … …………………… ∞ … Exact solution

27. 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).