# [DFTB-Plus-User] regarding forces module

Sharma SRK Chaitanya Yamijala sharmajncasr at gmail.com
Thu Aug 24 21:20:51 CEST 2017

```Dear Developers,

Would it be possible to you to explain why there is a need for the loop
listed below while calculating nonscc derivatives?

!! Add contribution to the force from atom 1 onto atom 2f using the
!! symmetry in the blocks, and also the skew symmetry in the
!! derivatives
do ii = 1, 3
deriv(ii,iAtom2f) = deriv(ii,iAtom2f) &
&- sum(sqrDMTmp(1:nOrb2,1:nOrb1) &
&* 2.0_dp*hPrimeTmp(1:nOrb2,1:nOrb1,ii)) &
&+ sum(sqrEDMTmp(1:nOrb2,1:nOrb1)&
&* 2.0_dp*sPrimeTmp(1:nOrb2,1:nOrb1,ii))
end do

I read the comments above the loop. But, it is not clear to me from these
comments that why the first loop in the derivative_nonSCC routine, i.e.,

do ii = 1, 3
deriv(ii,iAtom1) = deriv(ii,iAtom1) &
&+ sum(sqrDMTmp(1:nOrb2,1:nOrb1)&
&* 2.0_dp*hPrimeTmp(1:nOrb2,1:nOrb1,ii)) &
&- sum(sqrEDMTmp(1:nOrb2,1:nOrb1)&
&* 2.0_dp*sPrimeTmp(1:nOrb2,1:nOrb1,ii))
end do

is not sufficient to describe the force on iAtom1?

Based on my understanding, the force on each atom should be related to the
summation (of rho_uv * H_uv and rhoE_uv * S_uv) on all the orbitals related
to the atom of interest and the orbitals corresponding to all the other
neighbouring atoms and I could clearly see that this information is already
considered in the first loop.

Kindly, help me to understand.

Sincerely,
Sharma.

P. S.: May I also know, why derivative_nonSCC is separately made public
despite its availability through the derivative_shift interface?

-------------------------------------------------------------------
Sharma
http://www.chem.rochester.edu/groups/huo/people/
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