The ABINIT Users Mailing List ( CLOSED )

["Aldo Humberto Romero" <aromero@qro.cinvestav.mx>]

Dear Cheti


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

Ok... sounds very simple, 2 atoms per unit cell.

->
-> 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)

did you check some of the points before the transition ? to see if the volume
was correlated with the pressure? are you sure that the fitting was
correct? one of the
usual mistakes is to take the wrong units during the fitting..

->
-> 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.

It seems to me that this could be a problem of units during the fitting..
->
-> What I do for example in order to plot frequency vs pressure .what
-> pressure
-> chosen.

Well.. the real pressure is the one you obtain from the calculation... but
for most cases, it is close to the one you obtain from the fitting.. if
you want, you can
send me the data volume vs energy and I can do the fitting for you..
and see if it is comparable to waht you are getting..


regards

-aldo.


-> Best regards
->
-> 2009/12/24 Aldo Humberto Romero <aromero@qro.cinvestav.mx>
->
->> Dear Cheti
->>
->> please, next time provide more info about the calculation you have
->> performed.
->> Is CaS a Zb? is a Wz? is there any inner coordinates? did you relax the
->> system
->> with the same parameters ? did you check the forces and
->> stresses after relaxation? is there anything different? please provide
->> more details such
->> we can try to find out where your problem is.
->>
->> Best regards
->>
->> -aldo.
->>
->> -> I new user with abinitio method :
->> ->
->> -> 1-I studied the effect of pressure of lattice dynamic  of  CaS, so  I
->> -> calculated the lattice constant and bulk modulus  used  Murnaghan
->> equation
->> ->  of
->> -> state. when I calculated the specter of the phonon at  equilibrium
->> lattice
->> -> ,
->> -> in trf2_1 the pressure is -4.8 GPa.
->> ->
->> -> Why the pressure not  0GPa.
->> ->
->> -> 2- in order to calculated the  frequency vs pressure. from  eos-fit
->> for
->> -> example the volume at transition pressure  (40GPa) is (b0) , so when
->> I
->> -> calculated the specter phonon  in trf2_1.out  the pressure  is 25
->> GPa.
->> -> What
->> -> I do ?
->> ->
->> -> And what pressure I used to plot the phonon vs pressure.
->> ->
->> ->
->> ->
->> -> Best regards.
->> ->
->>
->>
->> --
->>
->> Prof. Aldo Humberto Romero
->> CINVESTAV-Unidad Queretaro
->> Libramiento Norponiente 2000
->> CP 76230, Queretaro, QRO, Mexico
->> tel: 442 211 9909
->> fax: 442 211 9938
->>
->> email: aromero@qro.cinvestav.mx
->>        aldorome@gmail.com
->> www: qro.cinvestav.mx/~aromero <http://qro.cinvestav.mx/%7Earomero>
->>
->>
->


-- 

Prof. Aldo Humberto Romero
CINVESTAV-Unidad Queretaro
Libramiento Norponiente 2000
CP 76230, Queretaro, QRO, Mexico
tel: 442 211 9909
fax: 442 211 9938

email: aromero@qro.cinvestav.mx
        aldorome@gmail.com
www: qro.cinvestav.mx/~aromero