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<div>Dear Wang, H:
<br>
<font color="#444444">> Suppose a uniaxial crystal(no cubic): c>a=b. Suppose there is a
<br>
</font><font color="#444444">> double-degenerate polar E_u mode, in which all atoms move in the a-b plane.
<br>
</font><font color="#444444">> If the wavevector around Gamma point is in the a-b plane, the degenerate E
<br>
</font><font color="#444444">> mode will split into LO and TO mode.
<br>
</font><font color="#444444">> Am I right?
</font></div><div><font color="#444444"><br>
It is probable for some ionic crystals, but still depends.
<br>
Maybe the Equ. 98 (nonanalytic part of the dynamical matrix) in Rev. Mod. Phys. 73, 515 (2001) by prof. S. Baroni etc. and the related parts in the book "Dynamical Theory of Crystal Lattices" by M. Born & K. Huang can give you much information if these things are important/interesting to you.
<br>
<br>
In addition, is the method "finite difference" you mentioned exactly the "frozen phonon" or "small displacement" method? If so, the treatment like that in PWSCF should be good to obtain a satisfied LO/TO splitting, although honestly I haven't done this in the "frozen phonon" method. Some people propose other methods, like using a prolonged supercell, which I think, could not fully describe the long-range dipolar interaction, because of its long-range character (as prof. Stefano de Gironcoli said ~1/R^3).
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<br>
Best Wishes!
<br>
Yours Sincerely
<br>
L. F. Huang
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<br>
L.F.Huang(黄良锋) DFT and phonon physics
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<br>
Add: Research Laboratory for Computational Materials Sciences,
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Instutue of Solid State Physics,the Chinese Academy of Sciences,
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P.O.Box 1129, Hefei 230031, P.R.China
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Tel: 86-551-5591464-326(office)
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Fax: 86-551-5591434
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Our group: <a href="http://theory.issp.ac.cn/" target="_blank">http://theory.issp.ac.cn</a>
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