Geometries are defined in terms of the atoms involved and their coordinates.
If you are new to MOPAC, then the fastest way to understand the general layout
is to look at an example of a simple data set.
The data is in a "free-format" This means that integers and real numbers can be interspersed, and numbers can be separated by one or more spaces, a tab and/or by one comma. If a number is not specified, its value is set to zero.
The geometry can be defined in terms of either internal or Cartesian coordinates, or a mixture of the two, or Gaussian format.