[Pw_forum] equation of state fitting

[Michael Mehl]

For a *numerical* differentiation you need E(V) at least at three separate
volumes.  The reason we use equations of state is that no one wants to do
numerical derivatives if they don't have to.  I suggested the numerical
differentiation as a check on the bulk modulus you get from the equation of
state.

As for the rest, I said in my experience the bulk modulus isn't off that
much.  Long ago we did this for various elements within the LDA:
https://doi.org/10.1103/PhysRevB.54.4519

But I realized there is an important question we didn't ask: what system
are you looking at.  For some elements, especially the alkali metals, DFT
is difficult.



On Wed, Aug 16, 2017 at 2:52 PM, Sunetra Das <sunetra.das.88 at gmail.com>
wrote:

>
> Dear Dr. Michael Mehl,
>
> Quoting you from your reply, "You must have minimum energy structure for
> your crystal structure at that volume.  This means minimizing the energy
> with respect to all atomic positions and all lattice parameters: b/a, c/a,
> etc., while keeping the volume fixed."
>
> --

[image: USNA_Gold_Seal.png]

Michael J. Mehl, Ph.D.

Kinnear Chair in Physics

The United States Naval Academy

Mail Stop 9C

572 Holloway Road

Chauvenet Hall 257

Annapolis MD 21402

(410)293-6685

mmehl at usna.edu

Library of Crystallographic Prototypes <http://aflow.org/CrystalDatabase/>
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