[Pw_forum] Cohesive energy

[Riccardo Sabatini]

Hello everyone,

    i have a couple of questions about cohesive energy I'd like to post 
here at the list. I want to calculate the cohesive energy for the Si in 
diamond structure. For what i understood the CE (cohesive energy) is the 
difference between the energy at atoms free and the energy of atoms 
bounded in the structure you want to study.

    My first question is about free energy for reference. To evaluate 
the Si energy alone i made a easy run with one atom in a supercell, big 
enough to delete all the other atom's effects. The result i had is 
around -7.47 Ry. My first question is about this result. I'm following a 
tutorial presentation  
(http://mpdc.mae.cornell.edu/Courses/MAE715/Symposium/Veera-Presentation.ppt) 
and there is listed a value of -7.52 Ry for the Si atom free. I tried 
with different pseudo potentials and with bigger and bigger cells but i 
can't reproduce that result. I think this difference could arise from a 
different pseudopotential used by the author and here comes  my 
question, is there a reference database for pwscf calculations with the 
pseudopotentials used by default ? I tried to search but the NIST 
database i think uses all-electron's calculations (Si atom results to 
have -288 hartree) and so is not very useful for a fast check...

    Anyway, i did the run for the Si in diamond structure.  I tried two 
ways, one with a fcc structure with 2 atoms in the cell and another with 
a sc cell with 8 atoms. The results seem correct but i don't understand 
what result should i take. In the tutorial (the one cited above) there 
is a value called energy/atom and energy/cell. In the first case i 
suppose one has to divide the total energy by the numbers of atoms in 
the cell (2 in the first case and 8 in the second) but i can't figure 
out what is energy/cell (i tried to divide by the volume but the results 
are order of magnitude different).
    Is it correct to define a energy for each atom in a cell in this way 
? (If i remember correctly the difference in energy for the sc 
simulation and for the fcc arise from the difference in volume, one is 4 
times than the other not from the numbers of atoms). If yes, i tried to 
compare the energy/atom for the fcc diamond simulation and for the sc 
diamond simulation and the results are a little different, can this be 
caused by the number of plane waves used ?

    Many thanks for the help and for this mailing list, best regards

                Riccardo