[Pw_forum] Interpretation of vc-relax output

[stefano de gironcoli]

if nor celldm(1) [in Bohr] nor A [in \AA] are given in input and the 
CELL_PARAMETERS card is given then alat is defined as the length (in 
bohr) of the first vector in CELL_PARAMETERS
in your case 7.5*sqrt(2.0)=10.60660172 bohr

if celldm(1) or A are given then the CELL_PARAMETERS are interpreted as 
given in those units.
In input atomic positions are given in the selected units, in output 
they are always given in unit of the alat value computed/defined in input.

If you want to use AA as your input/output length unit the easiest way 
is to define

A=1.0 or celldm(1)=1.889726878 in the &system namelist
CELL_PARAMETERS
...
ATOMIC_POSITIONS Angstrom [or alat since they are the same]
...

in output the positions and cell information will be given in (input) 
alat units, that is in AA again.

HTH

stefano

PS
in your case the silicon cell contracted by
0.487685488 * alat / 7.5 = 0.487685488 * sqrt(2)
the final cell corresponds to an FCC with
final_alat = input_alat * 0.487685488 / 0.5 = 10,345371472 bohr

In these units the atomic coordinates are just
0 0 0 and 1/4 1/4 1/4 (plus a small shift away from the origin due to 
the fact that the center of mass remain fixed during relaxation)



On 21/12/2015 07:30, Ridwan Agbaoye wrote:
> Good Morning Everybody,
> I need some help with the interpretation of vc-relax output and 
> conversion from  alat to bohr
> I want to do vc-relax calculation for a monoclinic system but i decide 
> to start with silicon a face-centered cubic system to understand the 
> concept of vc-relax properly
> I tried vc-relax for silicon and i got the following output
> Begin Final Coordinate
> New Unit Cell volume = 276.90769au^3
> CELL PARAMETERS(alat=10.60660172)
> -0.487685488  -0.000000000  0.487685488
>  0.000117363  0.487685488    0.487685488
> -0.487685488  0.487685488  -0.000000000
> ATOMIC POSITIONS(alat)
> si    -0.035697193  -0.035697193  -0.035697193
> si    0.208182292    0.208182292    0.208182292
> End of Final coordinate
> My input file has
> ibrav=0    nat=2    ntyp=2
> CELL PARAMETER cubic
> -7.5000  0.0000  7.5000
> 0.0000  7.5000   7.5000
> -7.5000  7.5000   0.0000
> How exactly do we derive our celldm(1) and how do we convert to bohr
>
> Do we need to convert the atomic position from alat to Bohr or Angstrom
>
> must i use bohr as the unit of CELL PARAMETERS or i can use Angstrom 
> and declare it as CELL PARAMETER(angstrom)
>
>
> Agbaoye Ridwan Olamide
> Federal University of Agriculture, Abeokuta
> M.Sc.(in view)
> agbaoyeridwan at yahoo.com
>
>
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