This keyword is found in the second and later automatically generated job steps for compound calculation like Opt Freq. It serves to ensure that all basis set, ECP, and fitting set information (as applicable) is read from the checkpoint file while retaining the keywords for internally stored components (regardless of whether they were modified in the route) so that they can be reported in the output and archive entries for these later calculations. This keyword serves no purpose within route sections created by users.
The following output from the second job step of an Opt Freq calculation illustrates the GenChk keyword:
Link1: Proceeding to internal job step number 2.
-------------------------------------------------------------
#N Geom=AllCheck Guess=Read SCRF=Check GenChk RHF/STO-3G Freq
-------------------------------------------------------------
