How to analysis CalculateAtomicForces

[fangyongxinxi]

I used the usage example, on page 285 in the manual, to test how to use CalculateAtomicForces, and got this answer:

     Fx             Fy             Fz
o    -0.0247      -0.0319       0
h    0.0308      0.0013          0
h    -0.0064       0.0303        0

As for the whole h2o, the sum of these forces is ok, but for an certain atom, it is not like that. How to analysis the answer?　And, I want to use CalculateAtomicForces to measure an certain atom in TwoProbeSystem under different bias, is that ok? Thanks for your answer.

[zh]

In DFT simulation, we usually require that the force exerting on atoms are less than a certain value (i.e, if the force on each atom  reaches a converge criterion). It's meaningless to optimize the structure to make the force be zero for each atom. For the two-probe system, the forces calculated by the NEGF formula is still argued in science and they may be not  reliable. 

The optimization of two-probe system in ATK is indeed converted into the optimization of an equivalent bulk system. The "CalculateAtomicForces()" can not be used to optimize the two-probe system under different bias.