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I am trying to perform DFT calculations for CsPbBr3 system on QE v7.2. To exploit symmetry i have given the space_group=221 and A=5.95(angstroms) in input file for vc-relax calculation. The following CELL_PARAMETERShave been generated in output before the calculation ends.

CELL_PARAMETERS (alat= 11.24387044)
1.007708030   0.000000000   0.000000000
0.000000000   1.007708030   0.000000000
0.000000000   0.000000000   1.007708030

The alat= 11.24387044 is in a.u. In the vc-relax calcualtion, the alat= 11.24387044 value remains the same however, the value 1.007708030 has changes for each scf cycle. For my next calculations, should I use the lattice parameter alat= 11.24387044 as it is or11.24387044*1.007708030 before converting it into angstroms?

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  • $\begingroup$ +1 but please do not make new tags (see my edit). $\endgroup$
    – Nike Dattani - No Free Time
    Commented Apr 28 at 11:03
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    $\begingroup$ You can use A = 5.95*1.07708030 if you are more familiar with that $\endgroup$
    – naturallyInconsistent
    Commented Apr 28 at 11:50

1 Answer 1

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alat works like a scale factor. So, as pointed out by naturallyInconsistent, you can use A=5.95*1.07708030 because in QE, A is in Angstrom. If you want to use 11.24387044*1.007708030, then use the celldm(1) instead of A since celldm(1) takes the value in Bohr radius, i.e., the a.u. unit. However, note that QE doesn't parse math expression (unless you are using pwtk). So, instead of using a*b type of expression, use c where c=a*b. In summary, for your case, either use A=6.40862779 or celldm(1)=11.3305385.

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