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

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

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