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Hello everybody,

When  read the paper 'Lattice dynamics and dielectric properties of TiO2 
anatase: A first-principles study' published PHYSICAL REVIEW B 66, 155213 
(2002).  there is one point I can not undserstand, i.e., why there is a LO-TO
splitting for the IR active A2u?

To my best of knowledge, A2u is only a one dimmensional representation for
group D4h(4/mmm). It is quiet easy to find the LO-TO splitting  for Eu
representation in ABINIT calculation, because it is a two dimmensional
representation. so it could have the LO-TO splitting. And the same A2u
Splitting also exists in article 'Lattice dynamics and Dielectric Properties 
of
SiO2 Stishovite' published on PRL, vo72, p1686,1994.Why there is a LO-TO
splitting for the IR active one dimmenstional representation A2u mode?  Could
it be splitted in optical measurement?  

I will appreciate your help.

Best,

T Gao