From: Giacomo Fiorin (giacomo.fiorin_at_gmail.com)
Date: Mon Jan 22 2018 - 08:31:45 CST

Hi Ern, the convention arises from the fact that when a molecule is not at
its most stable ionization state, it will lose or acquire electrons until
it reaches that state.

For most compounds that are eventually modeled with classical force fields,
spin = 0 or multiplicity = 1 is the equilibrium state. But there are also
many exceptions that you should watch for, because a mistake in the
equilibrium spin would lead to force field parameters that are essentially
junk.

A notable example are paramagnetic spin labels, which are used in magnetic
resonance specifically because of their unpaired electron.

Giacomo

On Mon, Jan 22, 2018 at 3:14 AM, Ern Ong <Ern.Ong_at_student.adfa.edu.au>
wrote:

> Hi VMD users,
>
>
>
> The input of a spin multiplicity value is required in Opt. Geometry, Water
> Int. and Scan Torsions. Regarding to the determination of a correct spin
> multiplicity value, where M = 2S + 1, there are several people suggested
> that as a rule of thumb, different values of spin multiplicity should be
> run (may be from 1 to 7) and select the one that provides the lowest value
> of nuclear repulsion energy (in Hartrees) as reported in the Gaussian
> output file.
>
>
>
> Can someone please comment on this convention?
>
>
>
> Thank you.
>
>
>
> *Best Regards,*
>
>
>
> Ernest Ong Ern Seang
>
>
>
> *PhD student*
>
> *School of Engineering and Information Technology (SEIT)*
>
> *University of New South Wales@ ADFA*
>
> *NorthCott Drive, Campbell*
>
> *ACT 2600, Australia.*
>

-- 
Giacomo Fiorin
Associate Professor of Research, Temple University, Philadelphia, PA
Contractor, National Institutes of Health, Bethesda, MD
http://goo.gl/Q3TBQU
https://github.com/giacomofiorin