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Why we need to optimise

  • The most stable energy structure of the molecule is the minimum energy structure of the molecule
    • The nature of matter is that it will always do whatever is energetically favourable to end up in the lowest energy state
  • Structure ALWAYS dictates properties
  • Isomer determination can be carried out by looking at a molecule’s relative energies

Structure obtained

  • The geometry R at the global minimum energy \(V - (V(R))\) is the optimised geometry
  • The local minima is another stable isomer

QM Model

  • Are always written as theory/basis
    • The theory being the methodology/calculation set used to obtain the results
    • The basis set being the functions chosen to describe \(\psi\) of \(e^−\)
  • In property calculations, we use two sets of methodologies to describe the model

theory/basis // theory/basis

  • Where the first set of methodology refers to the property calculation itself and the second refers to the optimisation methodology
    • This is because we’ll usually run property calculations in conditions that we won’t have originally optimised for
  • Model1 is usually bigger than model2, because the optimisation is typically the heaviest computational process.
  • It’s important to decide on your model before you start any calculations, as the logic you use to decide this is really theoretically based
  • Considerations involve:
    • What is my computer capable of
    • What properties do I need to obtain
    • How big is the molecule
    • Do I need to account for (each can vary in complication or importance of the issue as well):
      • Time dependence
      • Correlation
      • Core \(e^−\)
      • Charge
      • Polarisability
      • Solvation
  • Models and basis sets are usually paired to obtain specific results, with different combinations being better for different things
    • Basis sets themselves vary in both accuracy and ease of calculation, with some basis sets being far more accurate, but being harder to compute as a result, regardless of function count
  • The specific theory used can also be a huge factor in the accuracy of any calculation