On 03/08/2015 08:01, Umesh Roy wrote:
Dear Stefano,
Yes we can identify the type phonon modes by looking at the
eigen vectors whether it is LA or TO. To clarify my question here in
the following I attached the input file for matdyn.x to compute the
phonon frequencies from interatomic force contant at any orbitrary q
vectors. I have taken a fixed q vector of magnitude |q|=0.4 in the
units of (2*pi/a) and say a number of directions of 196. I calculated
phonon frequencies along all these directions. For small group of q
vectors we know that first three modes are acoustic modes and the
higher modes are optical modes. As we increase magnitude of q vectors
we can have optical modes lower than the acoustic modes in magnitude
in a specific direction. As we see that the phonon frequencies are
written in the file *fe.freq* increasing order. My question is if they
are printed as increasing order then how do we know which frequency
belongs to acoustic branch and which frequency belongs to optical branch?
you don't.
or rather define what you mean by acoustic and apply the definition. :)
I can think of at least two definitions and they differ, so it's up to you.
It may be useful to ask yourself WHY you want to separate the acoustic
branches from the others.
stefano
Thank you.
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