[Pw_forum] need help about incorporation energy calculation

[Shaofeng Wang]

Dear pwscf experts,

I am doing some calculations about incorporation of arsenate ions into 
lattice of gypsum. But I am not clear how to calculate the overall reaction 
energy. For example, I calculated the total energy of gypsum supercell 
(2*1*2) and arsenate incorporated gypsum supercell. But for AsO43- and SO42- 
ions, I just calculated the total energy of AsO4 with -3 charge and SO4 
group with -2 charge in a cubic box (15 * 15 * 15A). Then the overall 
reaction energy was calculated from the difference between the total energy 
of products and reactants as following equation.
Ca16S16O96H64 + AsO43- = Ca16AsS15O96H64- + SO42-

Did I do right thing?

If not, can any expert show me how to calculate correctly?

By the way, one more question is the lower total energy means more stable 
crystal?

Thanks in advance.

Best,

Shaofeng


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Shaofeng Wang, Ph.D of Geochemistry
Environmental Molecular Science Group
Institute of Applied Ecology, Chinese Academy of Sciences
Shenyang, 110016, China
wangshaofeng at iae.ac.cn
www.iae.cas.cn