Dear Francisco:as a general statement, your expectation is justified. However, 
unexpected things happen.Having posted an (input) / output file would help to 
pinpoint a problem. Without any additional information, I'd guess that your 
lattice vectors are not exactly fcc, or the atoms are not exactly at symmetric 
positions. (E.g., you think that you scalethe atom coordinates with lattice 
parameter, but in fact you don't).Best regardsAndrei Postnikov
----- Francisco Garcia <> a écrit :
>Dear Users
I performed a series of single point energy calculations on an FCC crystal by 
varying the lattice constant. I'm trying to generate data points to optimize 
the lattice constant

I was expecting the force on each atom to zero by virtue of crystalline 
symmetry (no internal degree of freedom). But to my surprise, the forces are 
not zero. Increasing the Mesh cutoff from 300 Ry to higher values didn't help 
either. How is it possible for the atomic forces in a crystal to be non-zero?

SIESTA is supported by the Spanish Research Agency (AEI) and by the European 
H2020 MaX Centre of Excellence (

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