A comparaison study between using the Primitive and the Conventional cell

 

In this tutorial, we will comparison between using a primitive cell or a conventional cell.

Primitive cell

FeO-prim.cell

%BLOCK LATTICE_ABC
ang    # angstrom units
   3.11   3.11   3.11
   60   60   60
%ENDBLOCK LATTICE_ABC

%BLOCK POSITIONS_FRAC
Fe   0.000000000000000   0.000000000000000   0.000000000000000 
O    0.500000000000000   0.500000000000000   0.500000000000000 
%ENDBLOCK POSITIONS_FRAC


kpoint_mp_grid 6 6 6

symmetry_generate

FeO-prim.param

task     SinglePoint          ! The TASK keyword instructs CASTEP what to do
xc_functional   PBE           ! Which exchange-correlation functional to use.
cut_off_energy  600 eV        !
nextra_bands : 24
opt_strategy    speed         ! Choose algorithms for best speed

Execution

$ mpirun -np 4 castep.mpi FeO-prim 

 

Conventional cell

FeO-conv.cell

%BLOCK LATTICE_ABC
ang    # angstrom units
   4.40   4.40   4.40
   90   90   90
%ENDBLOCK LATTICE_ABC

%BLOCK POSITIONS_FRAC
Fe   0.000000000000000   0.000000000000000   0.000000000000000 
Fe   0.000000000000000   0.500000000000000   0.500000000000000 
Fe   0.500000000000000   0.000000000000000   0.500000000000000 
Fe   0.500000000000000   0.500000000000000   0.000000000000000 
O    0.000000000000000   0.500000000000000   0.000000000000000 
O    0.000000000000000   0.000000000000000   0.500000000000000 
O    0.500000000000000   0.500000000000000   0.500000000000000 
O    0.500000000000000   0.000000000000000   0.000000000000000 
%ENDBLOCK POSITIONS_FRAC

kpoint_mp_grid 6 6 6

symmetry_generate

FeO-conv.param

task     SinglePoint          ! The TASK keyword instructs CASTEP what to do
xc_functional   PBE           ! Which exchange-correlation functional to use.
cut_off_energy  600 eV        !
nextra_bands : 24
opt_strategy    speed         ! Choose algorithms for best speed

Execution

$ mpirun -np 4 castep.mpi FeO-conv

 

Now , we will check the castep files for both calculations

FeO-prim.castep

Final energy, E             =  -1295.317594516     eV
Final free energy (E-TS)    =  -1295.483998211     eV
(energies not corrected for finite basis set)
 
NB est. 0K energy (E-0.5TS)      =  -1295.400796363     eV

-

-

Initialisation time =     39.35 s
Calculation time    =     20.92 s
Finalisation time   =      0.28 s
Total time          =     60.54 s
Peak Memory Use     = 3098424 kB 

 

FeO-conv.castep

Final energy, E             =  -5181.419951474     eV
Final free energy (E-TS)    =  -5182.053538003     eV
(energies not corrected for finite basis set)
 
NB est. 0K energy (E-0.5TS)      =  -5181.736744739     eV

-

-

Initialisation time =     25.16 s
Calculation time    =    138.78 s
Finalisation time   =      0.32 s
Total time          =    164.27 s
Peak Memory Use     = 3102764 kB 

Comparaison

We Know that the conventional cell (8 atoms) in FCC structure is 4 times the primitive cell (2 atoms) and we see that the Total energy of conventional cell ( -5181.419951474 eV ) is 4 times the Total energy of the primitive cell (-1295.317594516 eV).

The time of calculation of the conventional (164.27 s) is 2.66 times the time of calculation of the primitive cell (60.54 s). So It's better to use the primitive cell in calculation.