I have used both Gaussian09 and Gaussian16 and one thing that seemed unusual to me is that I was unable to enter any fractional number as one of the arguments for a keyword in the Gaussian input file.
For example, to control the step length of the optimization procedure, the keyword is
maxstep. So I can enter something like this:
#n opt=(maxstep=1) freq b3lyp/6-31G !Runs normally
Whereas Gaussian gives an error in reading the input file if I use a fractional number (float) as an argument of
#n opt=(maxstep=0.5) freq b3lyp/6-31G !Error
I have not seen this in any other softwares. For example, GAMESS and Orca both allow non-integer values for arguments to keywords that control the optimization step size.
Why does Gaussian not allow non-integer arguments to its keywords? (I haven't checked other keywords apart from maxstep, because most other keywords don't need floating point values, but the same problems seems to arise with maxstep for IRC)