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Semiempirical Methods

The big takeaway here for me is that semiempirical methods work by pre-screening the Fock matrix and using absolute values to replace the calculation of some of the more expensive integrals in the Hamiltonian

Two Electron Integrals

  • CNDO - Complete Neglect of Differential Overlap
    • Uses the ionisation potential and electron affinity to act as values in the electron repulsion term
    • Doesn’t differentiate between different orbitals on the same atom (no singlet-triplet splitting)
    • Doesn’t account for difference in energy between rotamers
  • INDO - Intermediate Neglect of Differential Overlap
    • Differentiates between different orbital shapes on the same atom
    • Are (often) useful for molecular spectroscopy in large molecules
  • NDDO - Neglect of Diatomic Differential Overlap
    • Uses INDO for electrons on the same atom
    • Uses a set of values for the 100 possible combinations of orbital/atom pairs for electrons on different atoms
      • Treats the interactions of different orbital shapes as point charge (s/s), dipole (s/p) or quadrupole (p/p)

Final Touch-ups

Pairwise Distance Directed Gaussian (PDDG)

Integrated in MNDO, AM1 and PM3, the nuclear repulsion term adds in some “gaussian ripples” that could be tuned to create repulsion or attraction at certain distances from the nuclei in question. This is an ad-hoc MM like fudging term.