# 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