[Pw_forum] more on variable-cell optimizations

[Lilia Boeri]

Dear PWSCF users,

I was wondering whether it is possible with PWSCF to optimize the unit 
cell while keeping the internal degrees of freedom (i.e. the reduced 
coordinates in the unit cell) fixed. 

If this is not possible, I would like to ask you the following question:
if I have, for example, an hexagonal system and I want to find how the c/a 
ratio evolves with pressure, I suppose I should calculate, for different 
volumes, the total energy as a function of c/a keeping the volume fixed. 
Then I would take the minimum point for each volume and with this build
a E(V) curve, differentiate and calculate the P(V) curve. 
Is it correct to expect that at the c/a value at which, for each 
volume, the total energy is minimum, the stress tensor should be isotropic 
? (i.e. the x,y,z components should be the same)?
I expect that if I want to reproduce or compare with an experiment done at 
isotropic pressure I should perform my calculations at some values at 
which the stress tensor calculated from first-priciples is isotropic... 
is the above procedure correct?
Thank you in advance for your response,
Lilia