[Pw_forum] Interpretation of vc-relax output

[Ridwan Agbaoye]

Does this answer work for all systems be it FCC or monoclinic or Hexagonal
"final_alat = input_alat * 0.487685488 / 0.5 = 10,345371472 bohr " where does 0.5 come from 

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  -0.035697193si    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 

Agbaoye Ridwan OlamideFederal University of Agriculture, AbeokutaM.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 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  
  
 _______________________________________________
Pw_forum mailing list
Pw_forum at pwscf.org
http://pwscf.org/mailman/listinfo/pw_forum 
 
 

  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20151223/7ee50dec/attachment.html>