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 PM3ROOT=4CISC.I.=4, none of these
keywords would be used by the OLDGEO calculation, unless they were
explicitly present in the OLDGEO keyword line.