$HESS group (relevant for RUNTYP=HESSIAN if RDHESS=.TRUE.) (relevant for RUNTYP=IRC if FREQ,CMODE not given) (relevant for RUNTYP=OPTIMIZE,SADPOINT if HESS=READ)
Formatted force constant matrix (FCM), i.e. hessian matrix. This data is punched out by a RUNTYP=HESSIAN job, in the correct format for subsequent runs. The first card in the group must be a title card.
A $HESS group is always punched in Cartesians. It will be transformed into internal coordinate space if a geometry search uses internals. It will be mass weighted (according to $MASS) for IRC and frequency runs.
The initial FCM is updated during the course of a geometry optimization or saddle point search, and will be punched if a run exhausts its time limit. This allows restarts where the job leaves off. You may want to read this FCM back into the program for your restart, or you may prefer to regenerate a new initial hessian. In any case, this updated hessian is absolutely not suitable for frequency prediction!