Acoustic phonons have zero frequency at X point in zincblende lattice

I am using GULP with a Stillinger-Weber plus Coulomb force field. I am mostly interested in the phonon dispersion curves and the lattice parameters. I’m doing calculations on ZnSe in the zincblende crystal structure. My force field gets the lattice constants almost spot on and the bulk modulus pretty close, and the phonon frequencies at the gamma point are very close to experiment. But when I calculate the phonon frequencies at the X point (lattice point 1 0 0), the acoustic phonons all have zero frequencies. The experimental frequencies are far from zero and I’ve looked up both experiments and calculations for other zincblende structures, and the acoustic phonon frequencies are far from zero at the X point. I’m trying to figure out whether this is a bug in GULP, or I’m misinterpreting the GULP input or output, or I’ve somehow come up with a force field that gets lots of things right but gets this particular thing very wrong.

From your description it sounds like it’s a matter of units for the BZ position. If the acoustic modes are zero then 1 0 0 is back at gamma & so 1 represents a complete k vector. Did you try 0.5 0 0? Note in GULP k points are by default with respect to the primitive cell BZ unless you specify the “kfull” keyword.

All of the other points along that direction, e.g. 0.5 0 0, give nonzero acoustic mode frequencies; however, 1 0 0 has significantly different frequencies for the optical phonons than does 0 0 0, so 1 0 0 is not just “back at gamma”. With regard to your second comment, I don’t have a very good understanding of reciprocal lattices so I need to think about that one some more! Thanks for the suggestion.

Zinc blende is in an F centred space group and so you definitely need “kfull” if you want the results in terms of the reciprocal lattice of the cubic cell, rather than the rhombohedral one.

Thanks! My phonon dispersion curves now make a lot more sense.