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[<ilukacevic@fizika.unios.hr>]

Dear Cheti,

Since you want to calculate phonons in the CsCl structure, which is cubic with
atomic positions fixed with symmetry, there is a ono-to-one correspondence
between the lattice constant and pressure. The pressure you get for a=6.04 in
CsCl in rf phonon calculation (in the end of first dataset) is perfectly
correct with a given set of input variables, ie. ecut, ngkpt, tsmear, ecutsm,
tol"something". If this set is equal in your total energy calculation and in
your rf phonon calculation (first dataset), then the pressure should be the
same.
You could converge this set of parameters in both kinds of calculation and 
then
do the both kinds of calculation with the stronger set.
If for some reason you don't want to use the same set, then you'll have to
guess the lattice constant in the rf phonon calculation for which you'll 
obtain
the exact pressure of 40 GPa.

Also, if you want to obtain the exact pressure of 40 GPa, you can always do
bfgs relaxation with fixed pressure at 40 GPa. Fitting with eos is only
fitting.

Cheers!

Igor Lukacevic


CaS crystallize in the NaCl structure at 0 temperature and 0pressure(B1phase)
and at 40GPa (good agreement with expt) crystallize in the CsCl structure.

1-The total energy of the b1-type phase and b2-type has been calculated at
different primitive cell volume and fitted to the murnaghan equation of state,
on the other hand, from (Murnaghan-EOS) the calculated enthalpy as function of
pressure  for CaS in b1 and b2 phase induced transition pressure is 40GPa.from
b2 phase the volume corresponding the transition pressure is 220 b3    →
(a(pt)=6.04 b)

2-in order to calculated the specter phonon at transition pressure in b2 phase
, I used  trf2-1.in from the input variable of the lattice constant is 6.04 ,
so in trf2_1. out  the pressure is not 40 GPa. On the other hand ,for other
lattice constant the pressure obtained from (m-eos) different to the imprinted
in trf2_1.out.

What I do for example in order to plot frequency vs pressure .what pressure
chosen.