[Pw_forum] Periodicity

[Yantao Wu]

Dear QE users,

I have a general question on how QE treats periodicity of the system, e.g.
a bcc lattice. I originally thought QE only calculates the wave function
for the system you input into the program and enforce the Bloch periodic
boundary condition of Psi(r+R) = exp(ikR)Psi(r) on the wave-functions,
where R, the lattice constant, QE knows from the input of ibrav. In this
case, QE doesn't "know" anything about the infinite lattice that may be
inferred from the input system.

But then I was told the other day that QE actually infers the infinite
lattice from the input system and calculates the wave-function for the
infinite system. Even though this sounds appealing to me, I'm still
confused by one seeming paradox. That is, if I double the size of the input
system but making sure that the infinite lattice inferred from this
double-sized system be the same as the original system, then the energy
calculated for the double-sized system will be almost precisely double of
the energy of the original system. This makes me feel like that QE treats
the system of interest to be just what the input system is.

Can anyone clarify how exactly is periodicity treated in QE?

Thank you much,
Yantao Wu,
Undergraduate Student,
Harvey Mudd College 15'
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