By now you should be familiar with the diagram below. As you may recall, there are two main decisions that you, the scientist, must make before submitting input files to computational chemistry programs. You must decide on the geometry of the molecule and on a starting set of mathematics and approximations known as the basis sets. Today we are going to deal with the first, the molecular geometry.
Every molecule has multiple geometries.
Choosing the correct molecular equilibrium geometry is very important when carrying out computational studies. This is because the energy of a molecule depends on its geometry. Even small changes in the structure can lead to drastic changes in total energy. The geometry of a molecule can be described using one of three different methods. The first is by using cartesian coordinates.