SCFCRT=n.nn

The default SCF criterion, 0.0001 kcal/mol, is to be replaced by that defined by SCFCRT=n.nnn. The SCF criterion is the change in energy in kcal/mol on two successive iterations. Other minor criteria may make the requirements for an SCF slightly more stringent. The SCF criterion can be varied from about 1.0 to 1.D-25, although numbers in the range 0.1 to 1.D-9 will suffice for most applications.

An overly tight criterion can lead to failure to achieve an SCF, and the consequent failure of the run.  See also RELSCF