If you check the SCF iterations with 2 different ISCF options, you may find that the energy in the first iteration are already different, even with the same initial guess.
But the overlap matrix of structures show that the structures are not orthogonal.
So I checked the overlap between determinants. See the attachments.
The calculations show that a lot of determinants are orthogonal to each other. I think this is the reason.
As we know, the ISCF=2 cannot handle the calculations in which structures or determinants are orthogonal, but ISCF=5 is able to do.
So you should NEVER trust the results provided by ISCF=2 in this case.
You may check the results by ISCF=1/3.
在 2015年9月26日星期六 UTC+8下午7:18:25，David Danovich写道：