NaF Berry Phase calculation

[tilak]

Dear VASP users and experts,

In "Berry Phase Type calculation" on NaF, if I use IGPAR=1, I have got:
For undistorted case:
-----------------------
e<r>_ev=( 0.00000 0.00000 0.00000 ) e*Angst
e<r>_bp=( 0.00000 0.00000 0.00000 ) e*Angst

Total electronic dipole moment: p[elc]=( 0.00000 0.00000 0.00000 ) e*Angst
ionic dipole moment: p[ion]=( 2.25510 2.25510 2.25510 ) e*Angst
----------------------------------------

For distorted case:
----------------
e<r>_ev=( 0.00000 0.00000 0.00004 ) e*Angst
e<r>_bp=( 0.00000 0.18033 0.18033 ) e*Angst

Total electronic dipole moment: p[elc]=( 0.00000 0.18033 0.18037 ) e*Angst
ionic dipole moment: p[ion]=( 2.25510 2.25510 1.93939 ) e*Angst
--------------------------------------
From these above two set of data, I can have "Born Effective Charge (BEC)": 0.9961864219 elect., consistent with manual.


BUT, If use IGPAR=3, I have got:

For distorted case, (undistorted case as same before):
------------------
e<r>_ev=( 0.00000 0.00000 0.00004 ) e*Angst
e<r>_bp=( -0.18033 -0.18033 0.00000 ) e*Angst

Total electronic dipole moment: p[elc]=( -0.18033 -0.18032 0.00004 ) e*Angst
ionic dipole moment: p[ion]=( 2.25510 2.25510 1.93939 ) e*Angst
-----------------------------------

If you look into the Total Electronic dipole moment, it has BP contribution, with same mag. but different sign, and If you use it for BEC calculation, it will be: -14.9967481117 elect., not same than before! How do I explain?

Thank you very much for your comments,
Tilak

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