[Pw_forum] Interpretation of vc-relax output

[stefano de gironcoli]

On 23/12/2015 11:53, Ridwan Agbaoye wrote:
>
> Does this answers work for all systems be it FCC or monoclinic or 
> Hexagonal
which part of the answer ?  the first part about definition of alat yes.
>
> "final_alat = input_alat * 0.487685488 / 0.5 = 10,345371472 bohr " 
> where does 0.5 come from
>
from the comparison of your final cell and the common definition of fcc 
Bravais lattice.

> If i am working on a monoclinic or Hexagonal crystal structure do i 
> still use the 0.5 or how do i calculate the 0.5
>
>   For the atomic position i got
>> ATOMIC POSITIONS(alat)
>> si    -0.035697193  -0.035697193  -0.035697193
>> si    0.208182292  0.208182292    0.208182292
> is there a rule to use in converting to
> 0 0 0 and 1/4 1/4 1/4
>
read and understand the documentation.
Ask yourself how would you have defined the atomic positions and check 
in a few cases that you understand the way the code does it. it's just 
geometry.
> Agbaoye Ridwan Olamide
> Federal University of Agriculture, Abeokuta
> M.Sc. (in view)
> On Monday, December 21, 2015 1:18 AM, stefano de gironcoli 
> <degironc at sissa.it> wrote:
>
>
> 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 remains 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 <mailto:agbaoyeridwan at yahoo.com>
>>
>>
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>

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