Chapter 1, Types of coordinates, z-matrices, input files 1A: The reaction coordinate and the potential energy surface The common one-dimensional plot, the two-dimensional surface, and the 3N-6 PES. 1B: Normal coordinates, first and second derivatives of the energy with respect to the (cartesian) coordinates; vibrations. 1C: Internal coordinates, bonds. angles and torsions as coordinates. 1D: Applying symmetry and dummy atoms in MOPAC input files, how dummy atoms and symmetry rules can greatly simplify the construction of z-matrices. 1E: Creating MOPAC input files, either from scratch or from within a modelling program. 1F: Creating Gamess input files from MOPAC output. This is the first chapter: Types of coordinates, z-matrices, input files Next chapter How to locate a Transition State.