Dear All,

I am trying to calculate phonon frequencies for a 2-D sheet with the help of DFPT. But the results show that the acoustic sum rule is violated i. e. there are finite values of phonon frequencies at the gamma point in the negative region.

Following are some of the input parameters in the INCAR file,

########################################################

ISTART = 0

INIWAV = 1

NWRITE = 2

LWAVE = .TRUE.

PREC = accurate

EDIFF = 1.0E-6

EDIFFG = -0.001

ISMEAR = 1

SIGMA = 0.001

ICHARG = 2

LCHARG = .TRUE.

ISYM = 1

IBRION = 8

ISIF = 3

NFREE = 2

NSW = 1

POTIM = 0.02

LPETIM = .FALSE.

LREAL = .FALSE.

LORBIT = 1

LPLANE = .TRUE.

LRPA = .FALSE.

###############################################################

The KPOINTS file is as follows,

###############################################################

Automatic mesh

0

Gamma

8 8 1

0. 0. 0.

##############################################################

To perform the phonon calculations, the structure is fully relaxed with ISIF=3 but the z-axis was kept fixed with the help of "selective dynamics" flag. The vacuum in the z-direction was set to 20 Angstrom. Further, the phonon calculations were performed on 2x2x1 supercell of the relaxed structure.

What could be the reason behind the incorrect results that I am getting?

Thanks in advance.

Best,

Deepashri.