Hi Hongyi,
Hongyi Zhao wrote:
...
I usually obtain the bulk modulus by fitting the Birch–Murnaghan
equation of state.
But it seems that the P = B \delta V / V is more simple in form than
Birch–Murnaghan equation of state. I've two issues about this method:
1- Comparison with the results with Birch–Murnaghan equation of state,
which is more accurate?
As far as I understand, Birch-Murnagan consists in plotting the energy
vs. lattice
parameter, draw a parabola through the points, and get the curvature;
right ?.
For that there's no need of anything but energy.
The above formula needs reliably converged forces (i.e. stresses), and
the involved
lattice parameter changes are typically 0.1 to 1 % so as to assure
you're in the linear
elastic regime. Moreover, better if you average the two runs +(\delta V
/ V) and
-(\delta V/V), in order to cancel the contribution of non-zero
remaining stresses of
the reference, equilibrium, box.
Provided the above is met both methods should match.
2- Can this method also be applied to other crystal structures other
than hcp?
Any crystal structures should do. But please bear in mind that bulk
modulus strictly
makes sense for isotropic bodies, or even cubic ones. For anything else
YOU MUST
KNOW WHAT YOU ARE DOING ;-).
Regards,
Roberto
