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[Yong Liu <occupierliu@yahoo.com.cn>]

Dear Abinitors:

      I've done the RF calculation about the elastic constants of tetragonal PbTiO3, Cij consists with the previous results of experiments and simulations except for C12, C44, and C66. Consindering the error induced from the k-point grid (involving 115 k-points) and numerical error (ecut 45hartree), the positive C12 and C66 are acceptable. But the negative C44 totally confused me. Or this negative elastic constant has some physical means?

P.S.

Cij (Relaxed ion, GPa)

   2.4239904   1.5071663   0.7684601   0.0000000  -0.0000001   0.0000000
   1.5071663   2.4239877   0.7684603   0.0000000   0.0000001   0.0000000
   0.7684603   0.7684601   0.5754473   0.0000000  -0.0000001   0.0000000
   0.0000000   0.0000000   0.0000000  -0.5976210   0.0000003   0.0000000
  -0.0000001   0.0000001  -0.0000001   0.0000003  -0.5976204   0.0000000
   0.0000000   0.0000000   0.0000000   0.0000000   0.0000000   1.0505907

Results from Journal of materials research, 12:2623-2627(1997)

c11 = 237Gpa,   c33 = 60Gpa,      c44 = 69Gpa
c66 = 144Gpa,   c12 = 90Gpa,      c13 = 70Gpa

Sincerely.
Yong Liu

-------------------------------------------------------------------------------------
Yong Liu,
Institute of Inorganic Materials,
Department of Materials Science and Engineering,
Zhejiang University,PR China
Doctoral student

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