[Wien] NON-ZERO forces for zinc-blend GaAs

[Stefaan Cottenier]

>I exchanged TOT by FOR in all *.in2* files
>and run 'run_lapw -ec 0.00005' again. Here is the result:
>
>marc-hn:~/wien_work/GaAs111APB/4atoms/noAPB> grep :FOR *.scf
>:FOR001:   1.ATOM      0.072      0.000      0.000     -0.072
>
>The problem is not gone
>  
>
Only now I notice that you did use -fc, as in your previous post you see 
the value for e.g. :FOR001 changing in the last iteration (thought all 
these lines were for one structure file, sorry):

>:FOR001:   1.ATOM      0.699      0.000      0.000      0.699
>
>:FOR001:   1.ATOM      0.603      0.000      0.000      0.603
>
>:FOR001:   1.ATOM      0.021      0.000      0.000      0.021
>
Two things about that:

1) Such a small force (this is mRy/au, not meV/Ang !) is essentially zero.

2) The difference betwee the final-minus-1 and final-minus-2 iterations 
is at the 0.1 level. That means that alos your final force is converged 
only upon that level (your new 0.072 value when you add one iteration 
confirms that). At that level of convergence, even the exact 0.000 lies 
within the numerical spread.

3) If you converge further, you will probably still find a force at the 
0.0x level (or perhaps a bit smaller). Increasing basis set size (up to 
even 9.0) would be needed to bring that one closer to zero. But once 
more, a force of 0.0x is zero in practice.

Stefaan


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