[QE-users] different alat values in data-file-schema.xml

[Pietro Delugas]

Hello

the input is generated following to the schema and  expects only lattice 
vectors in bohr units.

The presence of alat and bravais_index attributes is optional and 
indicates that the cell vectors were generated with the same algorithm 
used in the latgen routine.

In the case on ibrav = 0 cell vectors are arbitrarily defined by the 
user,  the routine which writes the xml file computes alat as the 
modulus of the first cell vector,  but it is probably better not to 
write it at all.

   In the other cases you mentioned the celldm(1) provided in input was  
probably equal to the modulus of the first cell_vector.  Or celldm(1) 
was not provided at all in which case alat is computed as the modulus of 
the first cell vector.

By the way in CELL PARAMETERS it is also possible to specify angstroms 
as units instead of alat and avoid to use celldm(1)


thanks for reporting the case

best regards

Pietro


On 05/10/2018 10:39, Ryky Nelson wrote:
> Hi all,
>
> some user gave me the following setup of the input file for the 
> namelist SYSTEM and the card  CELL_PARAMETERS
>
>  &SYSTEM
>                        ibrav = 0,
>                    celldm(1) = 1.8897265,
>                          nat = 4,
>                         ntyp = 3,
>                      ecutwfc = 70,
>                      ecutrho = 560,
>                         nbnd = 38,
>  /
> CELL_PARAMETERS {alat}
>    3.124623266  -3.124623266   0.000000000
>   -3.124623266   0.000000000  -3.124623266
>    3.124623266   3.124623266   0.000000000
>
> After finishing the calculation with QE 6.3 with the above setup, 
> strangely I got two alat values in data-file-schema.xml that are 
> different from each other; the first alat value, which is put inside 
> the section <input>, is 8.350483329220e0 and the other one, which is 
> inside <output>, is 1.889726500000e0 [which is equal to celldm(1)]. 
> Could any one tell me why they are different? My guess is that it has 
> to do with the CELL_PARAMETERS setup; since it's in {alat}, the 
> elements are supposed between -1 & 1, and previously, with other cases 
> using this [-1,1] setup in {alat}, I always got the same alat values 
> from the <input> & <output> sections? But Could any one tell me the 
> exact reason? Thanks!
>
> Ryky
>
>
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