Documentation on Firefly v. 8.0.0 numerical gradient features . The numerical gradient code is activated as follows: . ... The primary purpose of this code is to allow computations normally requiring analytic energy gradients, to be performed using those QC methods for which analytic gradients are not yet programmed. ... The numerical gradients code is tightly integrated into Firefly and can be used anywhere where the analytical energy gradients are required by default. ... The default is 2. ...