LCALCEPS, IVDW, TOTAL ENERGY, and BORN EFFECTIVE CHARGES

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LCALCEPS, IVDW, TOTAL ENERGY, and BORN EFFECTIVE CHARGES

Postby ido » Wed Oct 25, 2017 6:58 am

Hello,

I am using VASP to calculate the BORN EFFECTIVE CHARGES, with:

LCALCEPS = .TRUE.
EFIELD_PEAD = 0.0075 0.0075 0.0075

When I combined it with vdW corrections, either with TS or DFT-D2/D3, the final total energy writen in the OUTCAR or OSZICAR is:
TOTAL_ENERGY = TOTAL_ENERGY(SCF) + 2 * E_vdW
istead of just:
TOTAL_ENERGY = TOTAL_ENERGY(SCF) + 1 * E_vdW

It adds the vdW energy twice!
my questions:

1) Is this a bug?
2) If yes, what about the born charges. Are they ok? or they affected from this bug too?

Thanks from advanced!
Best regards,
Ido
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Re: LCALCEPS, IVDW, TOTAL ENERGY, and BORN EFFECTIVE CHARGES

Postby admin » Mon Oct 30, 2017 1:36 pm

1) Will be checked.
2) Born effective charges (BEC) depend only on the electronic structure.
Because vdW corrections do not change the electronic structure
calculated BEC are valid (the combination of of LCALCEPS=.TRUE.
and vdW corrections is not reasonable and will be blocked).
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Re: LCALCEPS, IVDW, TOTAL ENERGY, and BORN EFFECTIVE CHARGES

Postby ido » Tue Oct 31, 2017 5:38 am

Many Thanks, admin!!!

Regarding point 2), when vdW corrections will be blocked in BEC calculation, notice that it calculates forces differences F(E)-F(0) with respect to electric field E. So if the structure is vdW oprtimized than the vdW forces of the optimized structure should be included in the total forces F(0) (which are usually very small, below the threshold), as far as I understood it, since F(0) are the corresponding forces of the optimized structure with the vdW corrections.

Best regards,
Ido
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