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[z.wu@gl.ciw.edu]

I have sent an eratum to PRL for that paper. They made a few mistakes for the 
piezoelectric tensor.

The following is the comparison between ABINIT and LAPW, finite difference 
(FD) vs DFPT (LR), and GGA vs LDA. I used experimental crystal lattices.

unit: C/m^2

(a) clamp-ion (Proper)
------------------------------------------------------
         LDA             |       GGA
------------------------------------------------------
         LR      FD      |       FD     PRL80,4321
------------------------------------------------------
e33     -0.913  -0.911   |      -0.881  -0.88
e31      0.407   0.406   |       0.408  -1.72
e15      0.174   0.171   |       0.199   1.99
------------------------------------------------------

(b) relaxed-ion (Proper)
------------------------------------------------------
         LDA             |       GGA
------------------------------------------------------
         LR      FD      |       FD     PRL80,4321
------------------------------------------------------
e33      4.411   4.485   |       4.928   3.23
e31      2.058   2.070   |       2.263  -0.93
e15      6.625   6.361   |       5.975   3.15
------------------------------------------------------
LR: linear response; FD: finite difference.  

(c) Corrected answers for PRL (proper values, GGA, LAPW)
--------------------------------------------------------
         clamped-ion   |        relaxed-ion
--------------------------------------------------------
e33     -0.88          |         3.68
e31      0.35          |         1.88
e15      0.20          |         6.20
--------------------------------------------------------

Zhigang Wu

Geophysical Lab, Carnegie Inst. of Washington