Constants vs. geometric parameters

Note carefully the difference between constants defined with define and identifiers that are used for atomic coordinates in the data blocks of atoms and geovar. Constants defined under define are merely symbols for, and exactly equivalent to, certain numerical values, whereas the coordinate identifiers carry implications such as the distinction between frozen and optimization coordinates. Constants affect only the input after their definition and the location of their definition in the input file is significant. Geometric identifiers only relate to the data blocks of atoms and geovar respectively and the relative order in which the keys atoms and geovar occur is irrelevant.