Hi Andrei, Vasili
Vasili, MD.ConstantVolume is not what we want. In any method the volume
varies though only a small amount
[email protected] wrote:
Still, I don't understand the formula
P = B \delta V / V
in two aspects: 1) what is \delta? A difference? Between what and what?
\delta V is a small volume change, so that ( \delta V / V ) is the
volumetric
deformation ( trace of deformation tensor )
2) which pressure (uniaxial? hydrostatic? some mixture?) is to monitor
as one varies the volume, keeping c/a.
Pressure is what SIESTA calls as such, namely, -1/3 Tr(stress). Presently
it won't be hydrostatic, for sure.
About inner degrees of freedom in hcp 2-atom cell I still don't understand
either, nor about "distortion of the basal plane" as one uniformly scales
all the cell dimensions.
Let's say e_ij is the deformation tensor. Then every coordinate is imposed
the transformation
x'_i = 1 + e_ij x_j
Now, if relaxation is allowed and for a polyatomic basis, the above
relationship
won't generally be maintained: the different Bravais lattices attached
to each
atom of the basis will displace relative to each other. This is viewed
as an inner,
microscopic, degree of freedom, which is compatible with the "macroscopic"
deformation.
But again, this won't happen in the present case because no distortion
of the
basal plane is involved. In any case, it is always safest not to
constrain any
coordinate and let them do whatever they want.
Whatever; this is far from the Akshu's original question.
I don't know which is the situation about Tc, but in handbooks one most
likely
finds bulk and shear modulii ( or still worse, Young's modulus and
Poisson ratio )
as if the material was isotropic / polycrystalline. If that's the only
number you
have to compare with, then some sort of averaging your ab-initio
calculations
must be cooked.
So Akshu, why did you want bulk modulus in the first place ?.
Cheers,
Roberto
