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 (

Responder a