This Fortran binary file is a very detailed store of all of the information gathered during a calculation. The .check file is vital for checkpointing. When a job is resubmitted after an intentional or accidental interruption the information in the .check file is used to resume the calcultion from the end of the last successfully completed BFGS step. The parameters may be changed during the interruption. The line
continuation : default
must be included in the .param file to specify a restart and not a fresh calculation.