How do we get to a molecular model of geometry?

• Start with an approximate geometry, with atoms in roughly the right positions relative to each other:  modelling programs provide some means, usually graphical, to build molecules
• Need a  method of calculating an energy for a geometry, which will change as bond lengths and angles change:  the program does this calculation
• The program changes the bond lengths and angles in small steps, usually changing all of them a small amount in each step
• After each step, the program calculates the energy again
• The basic principle is:  insofar as the method of calculating the energy is a good one, then the geometry which gives the lowest energy, i.e. the most stable structure, is the best model geometry

• You can see an example of geometry optimisation, using Molden
• There are separate notes on the energy minimisation method and on  conformation searching