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Vdw-d3(bj) for 'band' calculation
Date: 2019/04/03 17:39
Name: Nikolai   <nilezay@gmail.com>

Dear colleagues,

My attempts to find equilibrium lattice constant of fcc silver with pbe+vdw-d3(bj) correction give rise to undefined vdw term, namely Uvdw=NaN. In case of d3(zero) calculation, Uvdw is 0.
Is it a bug? Or vdw-d3 scheme was implemented only for 'cluster', but not for 'band' type of calculation?
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Re: Vdw-d3(bj) for 'band' calculation ( No.1 )
Date: 2019/04/06 13:53
Name: Kunihiro Yananose  <ykunihiro@snu.ac.kr>

Dear Nikolai,

Hi. Did you install the patch 3.8.5? According to the README.txt document of patch, there was an error related with the division by 0 in DFT-D3, and it was fixed by the patch.

regards,
K. Yananose
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Re: Vdw-d3(bj) for 'band' calculation ( No.2 )
Date: 2019/04/08 15:25
Name: Nikolai  <nilezay@gmail.com>

Dear Kunihiro,

Thank you for response. Yes, I used the patch 3.8.5. Moreover, it is formally clear where the error comes from. In the following part of Total_energy.c

if (W>1.0e-12){
C6 = Z/W;
C8 = 3.0*C6*r2r4ab_dftD[wanA][wanB];
dC6ij[Gc_AN][Gc_BN]=((dZi*W)-(dWi*Z))/(W*W);
sqrtC8C6 = -1.0;
}
else{
C6 = 0.0;
C8 = 0.0;
dC6ij[Gc_AN][Gc_BN]=0.0;
sqrtC8C6=1.0;
}

In case of the second clause of if statement, just below in the code, the following expression gives Nan, because of C8/C6.

if (DFTD3_damp_dftD == 2){ /*DFTD3 BJ DAMPING*/
fdamp = (a1_dftD*sqrt(C8/C6)+a2_dftD);
....
}

It seems, nobody tried to optimize the cell with one atom using vdw-d3 correction, in case of fcc silver it has to provide the close to experimental value of lattice constant, i.e. its noticeable shortening in compare with pure PBE.

All the best,
Nikolai
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