[CPMD-list] multiplicity in a unit cell?

[Axel Kohlmeyer]

On Mon, 2 Jul 2007, Samir H. Mushrif wrote:

SM> Hello CPMD users,

hello samir,

SM>                   I am trying to simulate Palladium (II) 
SM> acetylacetonate [Pd(C5O2H7)2] which has following lattice structure:
SM> Pd atom is at the center of the molecule.
SM> Monoclinic symmetry
SM> a=9.9119 A, b=5.2232 A, c=10.3877 A, and beta = 95.807 degrees
SM> P 21/n space group
SM> Z=2
SM> [There is one molecule in the center of the cell and there is molecule 
SM> at every corner with Pd located exactly on the corners]
SM> 
SM> 
SM> Single Pd acetylacetonate molecule has MULTIPLICITY=3. When I am 
SM> simulating the unit cell (geometry optimization), do I have to take 
SM> MULTIPLICITY = 6? (I am asking this question since there are 9 Pd atoms 
SM> in the cell, though they are also shared by other unit cells and 

am i right to assume, that you have little experience with 
electron structure calculations? 

if you had both Pd atoms with two unpaired electrons each 
(i.e. multiplicity 2+1=3) pointed in the same direction, it would
be mulitplicity 5 (=4+1). however the total multiplicity of 
your unit cell depends on the magnetic properties of your compound. 
it could as easily be 1 (i.e. both Pd(II) cores have two spins up, 
but they are pointed in opposite directions => 0+1 ).

if you have the latter situation, it can become rather 
nasty rather quickly to get the right answer due to deficencies
in current DFT functionals. you may have to need to use a code 
that implements constraints to keep the spins in the right 
state or some self-interaction correction method, like DFT+U.

SM> overall each unit cell has effectively 2 Pd atoms.). Also I would like 
SM> to ask if I need to use FREE ENERGY FUNCTIONAL and k-points in the 
SM> simulation.

well, if you don't know this by yourself, i strongly recommend to
practice with a system, that is well known and where you can find
out by yourself, what you need to do (most likely, k-points, but
it is unlikely that you have fractional occupations).

SM> I have very little knowledge of crystallography so please help me in 
SM> this aspect and bear with me if what I am asking sounds very stupid.

no problem. it does not sound stupid. your questions are 
very valid for somebody who is new to this kind of calculations.

i would suggest you first have a look into books like 

Electronic Structure Calculations for Solids and Molecules: Theory and 
Computational Methods by Jorge Kohanoff (very good and pragmatic 
overview of theory and methods)

or 

Electronic Structure: Basic Theory and Practical Methods by 
Richard M. Martin (more complete and thorough, but not as readable)

which explain the basic theory, how to determine whether you get
useful results and possible pitfalls much better than any e-mail could.

hope this helps,
    axel.

SM> 
SM> Thanks a lot
SM> Samir
SM> _/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/_/
SM> 
SM> Samir H. Mushrif
SM> Department of Chemical Engineering
SM> Mcgill University
SM> 3610 University Street
SM> Montreal, Quebec H3A2B2
SM> Canada
SM> Ph: (Lab)514-398-5170
SM>     (Off)514-398-4169
SM> 
SM> 
SM> 
SM> 
SM> _______________________________________________
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SM> 

-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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