[Wien] total energy value too high
Paula R. Alonso
pralonso at cnea.gov.ar
Mon Oct 3 19:49:24 CEST 2005
Dear Paula & Stefaan,
I tried as you had suggested but the value derived is several orders of magnitude too high.
Should the derivation rather be :
Eformation = Total E (SiC) - Total E(Si) - Total E(C)
because one unit cell of SiC has one atom of Si and one atom of C.
In
Eformation = Total E (SiC) - .5 Total E(Si) - .5 Total E(C)
Total E is per atom. So if your SiC cell has two atoms, you divide by two its total energy value
If your cell for Si and C calculations has one atom, then you are multiply by 0.5 all terms.
In this case I derive value for a unit cell which I then convert to one mole by multiplying by the Avogadro number (N).
I ultimately need to derive gibbs free energy of formation (standard free energy of formation).
Which quantity should I be looking at?
You are obtaining enthalpy of formation at T=0K.
I would appreciate your suggestions once more.
Thank you very much in advance.
Chandrika
-----------------------------------------------------------------------------------------
Dr (Ms) Chandrika Varadachari
Raman Centre for Applied and Interdisciplinary Sciences
16A Jheel Road
Calcutta 700075
India
Tel : 91-33-24830029
Fax : 91-33-24180610
web : www.rcais.org.in
----- Original Message -----
From: Paula R. Alonso
To: A Mailing list for WIEN2k users
Sent: Tuesday, September 27, 2005 7:19 PM
Subject: RE: [Wien] total energy value too high
It means that you have to perform calculations for pure Si and pure C in the same structure. These values can not be derivated from the same calculations of the compound.
Then, for your SiC compound with 50at%C, you have Eformation = Total E (SiC) - .5 Total E(Si) - .5 Total E(C)
This is what you usually find in literature (for example C Colinet and A Pasturel, J Phase Equilibria vol 15 No 3 (1994)).
Paula.
----- Original Message -----
From: Chandrika
To: A Mailing list for WIEN2k users
Sent: Tuesday, September 27, 2005 1:58 AM
Subject: Re: [Wien] total energy value too high
Dear Paula,
Many thanks for the suggestion.
Does this mean that I should do total energy derivations for (i) a structure containing only Si (ii) a structure containing only C and (iii) subtract from the total energy for SiC.
Or are these values available within the derivations for SiC itself?
Thank you,
Chandrika
----- Original Message -----
From: Paula R. Alonso
To: A Mailing list for WIEN2k users
Sent: Monday, September 26, 2005 9:07 PM
Subject: RE: [Wien] total energy value too high
Free energy of formation is referred to the pure structures. You have to substrat from the total energy of the compound the total energy of the pure elements in the same structure, weighted by atomic fraction.
You obtain total energy in Ry/cell. You must first convert it to Ry/at. Then, 1Ry/at = 13.13 x 10^5 Joule/ mole .
Paula Alonso
Departamento de Materiales - Centro Atómico Constituyentes
Comisión Nacional de Energía Atómica
Avda Gral Paz 1499, San Martín, Buenos Aires, CP B1650KNA
Argentina.
----- Original Message -----
From: Chandrika
To: Wien at zeus.theochem.tuwien.ac.at
Sent: Monday, September 26, 2005 3:40 AM
Subject: [Wien] total energy value too high
Hello Wien Users,
I am trying to derive the total energy of SiC in order to later derive its standard free energy of formation. With all three options in Wien2k 05 (13, 14 and 5) I get values near -656 Ry. (i) I presumed that the value derived by Wien is Ry per unit cell. Is that correct?
(i) Therefore to convert to kcal/mole, I used 1Ry = 13.13 x 10^5 Joule/ mole . Is this okay?
(iii) The derived value of total energy of SiC is then -861300 KJoule/mole. This is several orders of magnitude higher than the free energy of formation of SiC which is -72.2 KJoule/mole.
Could anyone suggest where I have gone wrong?
I would particularly like to know about the conversion units.
Thanks in advance.
Chandrika
-----------------------------------------------------------------------------------------
Dr (Ms) Chandrika Varadachari
Raman Centre for Applied and Interdisciplinary Sciences
16A Jheel Road
Calcutta 700075
India
Tel : 91-33-24830029
Fax : 91-33-24180610
web : www.rcais.org.in
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