AMBER Archive (2008)

Subject: Re: AMBER: QM/MM and unpaired spin

From: Gustavo Seabra (
Date: Wed Dec 03 2008 - 07:53:00 CST

On Tue, Dec 2, 2008 at 10:42 PM, Ross Walker wrote:
> Hi Gustavo,
>> **In principle**, SCC-DFTB can deal with the restricted open-shell
>> case. But it hasn't been tested much, so you need to be careful.
> I think this will need some hacking of the code though. As it stands right
> now the value of spin from the mdin file does not appear to be used anywhere
> inside the DFTB code so I can't see how one would actually select the spin.
> Unless DFTB simply detects if there are an odd number of electrons and
> selects a doublet. This of course prohibits things like triplets etc.
> All the best
> Ross

Precisely. As I said, it can treat the *restricted open-shell* case,
and that one only. The user cannot select the spin. That comes from
the way DFTB populates the orbitals: roughly speaking, it simply
orders the orbitals by energies and put 2 electrons per orbital. If
the number of electrons is odd, then the last orbital will have the
extra electron. (The real procedure is slightly more complicated,
though, as it takes into account degeneracy and electronic temperature
for Fermi distribution.)

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