Subject: high-temp energy surfaces in magnetic systems

From: Ricardo Grau-Crespo <r.grau-crespo~at~mail.cryst.bbk.ac.uk>
Date: Fri, 09 Apr 2004 17:44:08 +0100

Dear all,

The energy hypersurface (energy as a function of the position of the 
nuclei) is normally considered
in molecular dynamics as a function that is independent of temperature. 
This is obviously just an approximation.
In particular, for atoms with non-zero magnetic moments (say, for 
example, a ferromagnetic material)
it is clear that the energy surface is not the same at low temperatures 
(in the ferromagnetic state) as
at high temperatures (paramagnetic state). In these magnetic systems, if 
one wants to use 0 K ab initio
calculations to obtain approximates 'high-temperature' energy surfaces  
(for example, in order to fit
interatomic potentials to be used in high temperature MD), one normally 
has to choose between:

1. spin polarized calculations - which introduce the error associated to 
the ordering of the frozen
magnetic moments (such order is not present at high temperatures), or
2. non spin polarized calculations - which introduce an error because of 
the wrong electronic
configurations of the magnetic atoms.

I would like to know which approximation is better, that is, which of 
these errors is less
significant for the HT energy surface. Is there a general answer for 
this question?
I would appreciate any comment or suggestion.  Thank you very much,

Ricardo.

-- 
Ricardo Grau-Crespo <r.grau-crespo~at~mail.cryst.bbk.ac.uk>
PhD Student - University of London
Christopher Ingold Laboratories, room 205
20 Gordon Street, London WC1H 0AJ
United Kingdom
Tel : +44(0) 20 7679 7465
Fax : +44(0) 20 7631 6803
Web : http://img.cryst.bbk.ac.uk/www/graucrespo/homepage.htm
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