Normally the program will stop if certain errors occur.  If GEO-OK is present, the job will continue, but there is an increased probability that the results will not be meaningful.  The errors that are affected are:

If two atoms are within 0.1 Ångstroms of each other.
In practice, most jobs that terminate due to these checks contain errors in data, so caution should be exercised if GEO-OK is used.

In MOZYME calculations, if the supplied charge is incorrect and GEO-OK is present, it will be replaced by the correct charge, and the calculation allowed to continue. In general, this is not a good idea: if a charge is supplied, and MOZYME predicts a different charge, examine the system to find out which is correct; don't simply assume MOZYME is right!

When comparing protein structures, a special keyword, GEO-OK+, is provided to allow tautomeric hydrogen atoms, i.e., hydrogen atoms that are on different atoms in the two structures, to be used in the comparison.  This is a special case, and should not be used routinely.