Setting parameters for calculating correct Hessian matrix

[maryam]

If any one can help me to set the parameters for calculating correct Hessian matrix or force constant matrix by vasp.4.6;

Some of my settings are as bellow:
INCAR file
IBRION=5
NSW=1
NFREE=2
POTIM=0.015

KPOINTS file
K-Points
0
Monkhorst Pack
15 15 15
0 0 0

But I have serious problems with the results of Hessian matrix in OUTCAR file:
1) main diagonal elements of matrix are negative;
2) some of its elements are zero but we expected non-zero values;


Thanks in advance for any help.

[physicalattraction]

There should be six elements zero (they coincide with the translation and rotation of the molecule/unit cell). When you calculate the frequencies with VASP, there is always the probability that one or a few of these frequencies are negative, due to numerical accuracy. If the bottom six are close to zero, your result is usually still trustworthy.

[admin]

please check whether the default value of EDIFF is sufficient to get reliable frequencies, you may need to increase the accuracy substantially (depending on the system you investigate)