question regarding kspace

Hi all:

I have a question regarding kspace modify in lammps, specifically about “ kspace modify slab” option. Considering the following 2 simulations with the same atomic configurations. The first one uses non-periodic boundary condition in z and kspace modify slab 3.0. The second one uses PBC in all directions but has a vacuum space above the sample in z direction so that lz_new = lz_old*v_factor. To let the two simulations yield the same energy, v_factor should be 3 or 4 ? Our result indicate that v_factor = 4. But looking at the source code we guess v_factor should be 3.

Thanks

Deena

Hi all:

I have a question regarding kspace modify in lammps, specifically about “
kspace modify slab” option. Considering the following 2 simulations with
the same atomic configurations. The first one uses non-periodic boundary
condition in z and kspace modify slab 3.0. The second one uses PBC in all
directions but has a vacuum space above the sample in z direction so that
lz_new = lz_old*v_factor. To let the two simulations yield the same energy,
v_factor should be 3 or 4 ? Our result indicate that v_factor = 4. But
looking at the source code we guess v_factor should be 3.

those two simulations are two different physical models, so the
energies are not comparable.
the requirement for tripling the volume in the non-periodic dimension
is a requirement for solving the poisson equation to an acceptable
accuracy to compute the dipole correction for decoupling the slabs.
making it larger can somewhat improve then accuracy in exchange for
additional computational effort, but the relative total energies have
no meaning in that respect.

axel.

Stan can answer this.

Steve

Did you add the shape correction term 2\pi*M/(3V) to your 3d test case?