I am performing relaxed scan in Gaussian (B3LYP/6-31G(d,p)). There are two small organic molecules, one of which is a radical, the other has a benzene ring.
I'm trying to understand the optimization output. Specifically, the line Optimization completed on the basis of negligible forces
. I understand that these forces are the maximum and RMS forces above.
Some examples of "negligible forces" from my output file:
Item Value
Maximum Force 0.000004
RMS Force 0.000001
Maximum Displacement 0.019917
RMS Displacement 0.004747
Item Value
Maximum Force 0.000002
RMS Force 0.000000
Maximum Displacement 0.002231
RMS Displacement 0.000490
Item Value
Maximum Force 0.000002
RMS Force 0.000001
Maximum Displacement 0.003132
RMS Displacement 0.000805
However, some small values like
Item Value
Maximum Force 0.000002
RMS Force 0.000001
Maximum Displacement 0.000666
RMS Displacement 0.000149
are followed only by Optimization completed
. So what does "negligible" mean? How small do they need to be to say they are negligible? Or is this connected to at least one NO
in convergence criterion section?
Next question, how reliable is optimization when such message pops up? If it's unreliable, what do I do about this? Would altering the initial geometry fix it when in only happens in a couple of scan steps?