[Pw_forum] how to calculate elastic constants

[Nicola Marzari]

Dear Adetunji,

> How can i calculate the elastic constants(C11,C12 and C44) at (0 K) and
> (0 GPa) of a semiconductor compound using pwscf ?.

by taking the second derivative of the energy with respect to the 
appropriate strain - it would be good to look at a book on elasticity
(e.g. Kittel) to clarify these concepts, and the Voigt notation (these
are 4th rank tensors, connecting strain and stress, but in a cubic 
material e.g. C12=Cxxxy)

> The second question:
> In studying the phase transition, we need to analysed the enthalpy
> variation with pressure. Where the enthalpy(H) = E + PV. So, how can
> this be implemented using pwscf?
> These are my questions.


You calculate E as a function of V, and for every calculation you get
P from the trace of the stress tensor, and construct H.
	
			nicola

-- 
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Prof Nicola Marzari    Department of Materials    University of Oxford
Chair of Materials Modelling  Director, Materials Modelling Laboratory
nicola.marzari at materials.ox.ac.uk     http://mml.materials.ox.ac.uk/NM