If multiple geometries are to be run, and the final geometry from one calculation is to be used to start the next calculation, OLDGEO should be specified. Example: If MNDO, AM1, and PM3 calculation, were to be done on one system, for which only a rough geometry was available, then after the MNDO calculation, the AM1 calculation could be done using the optimized MNDO geometry as the starting geometry, by specifying OLDGEO. After the AM1 calculation was done, then a PM3 calculation could also be done, starting with the old AM1 geometry, by again specifying OLDGEO.

When an OLDGEO calculation is started, none of the keywords from the previous calculation are assumed to be present.  Thus, if the first calculation had keywords PM3 ROOT=4 CIS C.I.=4, none of these keywords would be used by the OLDGEO calculation, unless they were explicitly present in the OLDGEO keyword line.