[Pw_forum] ?spam? Re: vc-relax - negative and non-zero cell parameters

[Максим Попов]

Dear Christopher,


The structure is chosen at random, as I simply wish to test the vc-relax
> procedure, so I don't expect it to have a particular symmetry, although in
> general, I am dealing with clusters of low symmetry anyway.
>
>
If you are dealing with clusters, then there is no utility of vc-relax for
you.



> My problem is that it makes no sense to me that off diagonal elements can
> be non-zero, when they are relative. How can one have a value 0.000341145 x
> 0 (in the case of element (1,2))?  This, I do not understand.
>
> Furthermore, what would a negative relative parameter mean?
> I suspect I am misinterpreting the meaning of these alat cell parameter
> matrices, but I'm not sure how.
>

The matrix, you see in the output, is Bravais matrix, i.e. a matrix, where
the cartesian coordinates of a
unit cell vectors are packed in rows.

The notation:

CELL_PARAMETERS (alat= 30.23000000)
 1.003485881   0.000341145  -0.000166335
 0.000341140   1.002476151   0.000847099
 -0.000166325   0.000846982   1.001936910

means that Bravais matrix is:

{ 1.003485881   0.000341145  -0.000166335  }
{ 0.000341140   1.002476151   0.000847099  }    times alat   =
{ -0.000166325   0.000846982   1.001936910 }

{   30.3354    0.0103   -0.0050 }
{    0.0103   30.3049    0.0256 }
{   -0.0050    0.0256   30.2886 }

Hope, it helps.

-- 
Best regards, Max Popov
Ph.D. student
Materials center Leoben (MCL), Leoben, Austria.
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