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)
[email protected]
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