[Pw_forum] phonon calculations in titanium

[Uri Argaman]

I have done phonon calculation in Titanium in both Alpha (non-ideal HCP)
and Omega (hexagonal with 3 atoms per unit cell) phases after relaxation.
When I done the phonon calculation with 2 q-points in each direction
(Monkhorst-Pack) I have got a reasonable phonon spectrum with q2r.x and
PlotPhon, but when I have done a phonon calculation with 3 q-points or 4
q-points in each direction I have got very large negative frequencies. I am
working with degauss=0.01 Ry or degauss=0.005 Ry because this is the
smearing in which the energy converge to 10^(-5) Ry. The energy is converge
to 10^(-6) Ry in respect to all the numerical parameters
(ecutwfc,ecutrho,k-points). When I increase the smearing to degauss=0.2 Ry
I have got reasonable phonon spectrum with wiggling. From calculations I
have done in zero Kelvin (only SCF without phonons) I found the phase
transition in zero Kelvin in reasonable pressure only with degauss<0.01 Ry.
To sum up, the cold curve looks reasonable with degauss<0.01 Ry but the
phonon spectrum looks reasonable only with degauss approximately 0.2 Ry.
What is your recommendation?

In addition I would like to ask if the q2r.x and/or PlotPhon apply any ASR
and what is the difference from matdyn.x in the sence that plotphon does
not use *.freq but only *.fc file?

I can put homogenous grid in the input file of matdyn.x. Is it causes any
problems with hexagonal lattice?
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