cell optimization strange values of the a, b, c vectors

[Jonathan Campeggio]

At the attention of the CP2K community,

I have done a cell optimization for a Pd 111 surface. From the experimental cif I have prepared a 111 unit cell. Then I have translated in the x y z directions of 2 3 1 respectively, using periodic boundary conditions.

In the attachment you can find input, output and the coordinates. I expected that the cell would "breath" in the 3 dimensions, while along x it is contracted. Is my idea wrong? Have I done mistakes in the input files.

Thank you for your competence and kindness.

Jonathan

[Marcella Iannuzzi]

Dear Jonathan,

You are simulating a bulk system, not a  (111) surface. 

In order to expose a surface, you should add vacuum in the direction orthogonal to the surface you want to expose, i.e., you should use a slab model.

The size 3x2x1 is rather small and probably not suitable to simulate the Pd(111) surface. Consider to increase the system size or use k-points.

Even if you do so, you can expect a relaxation of the topmost layers, but not a contraction in the lateral directions.

Regards

Marcella

[Jonathan Campeggio]

Dear Marcella,

 thank you for your kind reply. I understand your point.

Best regards,

Jonathan

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[Jonathan Campeggio]

Dear Marcella,

 I am very sorry to bother you again. As reported in the literature, I have built my slab with PBC along with x and y directions and I have put vacuum along z. The slab is small as you pointed out very correctly, but  I am doing some tests. Then I am running a Geometry Optimization of the slab with the last 2 layers completely fixed (XYZ), and allowing the first two breath along the z directions. As you can see in the position file (pd_slab-pos-1.xyz), the z constraint on the last two layers is not respected, despite it having been given in input. It appears that the second section &FIXED_ATOMS is not considered by the program. What is my error?

Thank you again for your competence and kindness,

Jonathan