[QE-users] fundamental questions about force convergence criterion

[ykhuang at dicp.ac.cn]

Greeting, QE users and experts,

    I am a little bit confused about force information printed by pw.x:

------START-------

     Forces acting on atoms (cartesian axes, Ry/au):


     atom    1 type  1   force =    -0.00001085    0.00000000    0.00000000
     atom    2 type  2   force =     0.00001085    0.00000000    0.00000000
     The non-local contrib.  to forces
     atom    1 type  1   force =    -1.19247240    0.00000000    0.00000000
     atom    2 type  2   force =     2.35869968    0.00000000    0.00000000
     The ionic contribution  to forces
     atom    1 type  1   force =    10.24429006   -0.00000000    0.00000000
     atom    2 type  2   force =   -10.24429006    0.00000000   -0.00000000
     The local contribution  to forces
     atom    1 type  1   force =    -9.05412227    0.00000000    0.00000000
     atom    2 type  2   force =     7.89944406    0.00000000    0.00000000
     The core correction contribution to forces
     atom    1 type  1   force =     0.00187326    0.00000000    0.00000000
     atom    2 type  2   force =    -0.01425971   -0.00000000   -0.00000000
     The Hubbard contrib.    to forces
     atom    1 type  1   force =     0.00000000    0.00000000    0.00000000
     atom    2 type  2   force =     0.00000000    0.00000000    0.00000000
     The SCF correction term to forces
     atom    1 type  1   force =     0.00000019   -0.00000000    0.00000000
     atom    2 type  2   force =     0.00000184   -0.00000000   -0.00000000


     DFT-D3 dispersion contribution to forces:
     atom    1 type  1   force =     0.00000264    0.00000000    0.00000000
     atom    2 type  2   force =    -0.00000264   -0.00000000   -0.00000000


     Total force =     0.000015     Total SCF correction =     0.000002


     DFT-D3 dispersion Force =     0.000004
 -------END--------

    (I just take optimization of CO molecule as example.) What make me confused are, 

    1. What do two lines below "Forces acting on atoms..." mean? I tried to add all "contribution" terms (from non-local to dispersion) of one atom together but found not to equal to 0.00001085.

    2. I find that "Total SCF correction" roughly equals to "1.9E-7 + 1.84E-6", and "DFT-D3 dispersion Force" roughly equals to "|2.64E-6|+|-2.64E-6|". However, I don't know how to obtain "Total force = 1.5E-5", I tried to sum over all terms of each atom above it, took their absolute values and add together, but result was 8.35E-4, not 0.000015 (1.5E-5).




    Any answers will be appreciated, thanks in advance!




Best regards!

Yike Huang, PhD candidate.

Dalian Institute of Chemical Physics, CAS, China.
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