2.3.3 External Symmetry

The external symmetry of a molecule represents the total symmetry of a molecule (taking into account a degree of free rotation). In the example presented, the procedure determines that propane has a symmetry of two because it matches the table where there are two sets of two matching ligands. In this case, the symmetry or the linearity of the ligands does not affect the total external symmetry. Within JTHERGAS, there are four symmetry specifications which yield a symmetry factor greater than one. These are given in the table. For example, using Table 4, 2,2-methylpropane has symmetry 12, C(NH₂)₄ has a symmetry of 4, propane has a symmetry 2, CH(CN)₃ has a symmetry of 3. For structures which multiple forms which could produce different symmetries assumptions have to be made about the 'average' or most prominent structure which may or may not have a symmetry contribution. This is a design decision which has to be made when creating the database.

Other similar symmetry tables are and can be defined for linear, doubly and triply bonded and saturated and unsaturated ringed structures. It can be that several tables (with structure) can match within a given target. The external symmetry is chosen to be the largest symmetry number.

For the determination of external symmetries, there are two sets of tables. The primary table is what has just been described. But a secondary table exists which describes the symmetry of the isolated ligands corresponding to the unspecified atoms.