Density Functional Theory is not guaranteed to give a bulk modulus that matches with experiment, unfortunately, though in most case I've tested it has been within 10%.
In addition to the equation of state approach, you can try computing the bulk modulus by numerical differentiation, using B(V) = V E''(V). Just be careful: the derivative is with respect to the volume V, not alat. 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. If your structure is simple enough you may not need to worry about this. On Wed, Aug 16, 2017 at 12:36 PM, Sunetra Das <[email protected]> wrote: > Dear QE users, > > I would like to know if there are any other equation of state that we can > use to fit the 'etot vs alat' graph to find out the equilibrium lattice > constant. I have been following the QE tutorials to learn the basics where > it has been demonstrated how to fit a etot vs alat graph using the four > given equations of state. But none of the obtained equilibrium bulk modulus > values from those equation fitting match the given experimental value, not > even close. > How to improve the fitting of the equations or is there any other equation > I can use to fit the obtained data? > > I am new to QE, any suggestion is highly appreciated. > > Thank you. > > Regards, > Sunetra Das > Assistant Professor, > MMC College, Kolkata, WB, > India. > > > _______________________________________________ > Pw_forum mailing list > [email protected] > http://pwscf.org/mailman/listinfo/pw_forum > -- [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 [email protected] Library of Crystallographic Prototypes <http://aflow.org/CrystalDatabase/>
_______________________________________________ Pw_forum mailing list [email protected] http://pwscf.org/mailman/listinfo/pw_forum
