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I am wondering if the negativity of elastic constant from ionic relaxation contribution is reasonable? If it is, does it mean the ionic relaxation mode is unstable?

I use M-P 888 Kpoints. The material is cubic BaSnO3, which is stable at 0 K.

Here is my INCAR and and OUTPUTS:

NSW = 1
ISIF = 3
IBRION = 6
ENCUT = 520
EDIFF = 1E-5
EDIFFG = -1E-8
ISYM = 0
ISMEAR = -1
SIGMA = 0.1

LPLANE = .TRUE.
NSIM = 4

LVTOT = .T.
KPAR = 4



SYMMETRIZED ELASTIC MODULI (kBar)
Direction XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
XX 2467.6439 711.4288 708.2952 -3.4668 2.3529 0.1122
YY 711.4288 2461.1179 709.1010 -3.4270 0.3129 2.3419
ZZ 708.2952 709.1010 2461.9007 1.4224 0.0945 0.3244
XY -3.4668 -3.4270 1.4224 910.0718 -4.0215 -0.0116
YZ 2.3529 0.3129 0.0945 -4.0215 900.1764 -4.0591
ZX 0.1122 2.3419 0.3244 -0.0116 -4.0591 900.1012
--------------------------------------------------------------------------------

ELASTIC MODULI CONTR FROM IONIC RELAXATION (kBar)
Direction XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
XX -0.2350 0.0134 0.0415 0.0068 0.0028 0.0152
YY 0.0134 -0.1952 0.0590 0.0003 0.0075 -0.0393
ZZ 0.0415 0.0590 -0.1998 0.0026 0.0006 0.0327
XY 0.0068 0.0003 0.0026 -0.0078 -0.0000 -0.0001
YZ 0.0028 0.0075 0.0006 -0.0000 -0.0078 0.0004
ZX 0.0152 -0.0393 0.0327 -0.0001 0.0004 -0.1603
--------------------------------------------------------------------------------

TOTAL ELASTIC MODULI (kBar)
Direction XX YY ZZ XY YZ ZX
--------------------------------------------------------------------------------
XX 2467.4089 711.4422 708.3368 -3.4599 2.3558 0.1274
YY 711.4422 2460.9227 709.1600 -3.4267 0.3205 2.3025
ZZ 708.3368 709.1600 2461.7010 1.4250 0.0950 0.3570
XY -3.4599 -3.4267 1.4250 910.0640 -4.0215 -0.0117
YZ 2.3558 0.3205 0.0950 -4.0215 900.1686 -4.0587
ZX 0.1274 2.3025 0.3570 -0.0117 -4.0587 899.9409
--------------------------------------------------------------------------------

Dear sjtuzhanglei,

The mechanical stability of materials based on elastic constants is usual cast in the form of satisfying certain inequalities. These inequalities depend on the crystal structure. See for example :
http://journals.aps.org/prb/abstract/10 ... .76.054115

So you cannot judge by observing a couple of negative constants.

Mostafa