[Q-e-developers] minimizzazione inefficiente

[Nicola Manini]

Hi Paolo,
	your comments made me curious, so I looked more into this problem.
I wrote a detailed report, whose conclusions are:
- PW computes extremely accurate forces.
- Total energies suffer from some nonrandom noise of unknown nature.
- The ``damp'' minimization algorithm has a bug showing near minima.

I attach the full report, and a bunch of files which could be useful to 
anyone who'd like to check and extend my findings. Enjoy!

Have a great, week-end, ciao,
						Nick



On 05/11/2011 08:46 PM, Paolo Giannozzi wrote:
>
> On May 9, 2011, at 16:33 , Nicola Manini wrote:
>
>> At Paolo's request, I attach the input file of a typical example
>> of problematic damp minimization
>
> I don't see any problem here: the system is already minimized.
> Really: forces are very small and energy is almost constant.
>
> ! total energy = -138.30526161 Ry
> ! total energy = -138.30526161 Ry
> ! total energy = -138.30526161 Ry
> ! total energy = -138.30526161 Ry
> ! total energy = -138.30526161 Ry
> ! total energy = -138.30526140 Ry
> ! total energy = -138.30526161 Ry
> ! total energy = -138.30526140 Ry
> ! total energy = -138.30526161 Ry
> ! total energy = -138.30526140 Ry
>
> I think that trying to further minimize is a waste of time. In
> principle minimization could go on forever, but I am afraid
> that at these levels, the noise on the energy and on the forces
> due to scf convergence, to the discreteness of the FFT grid
> (making the system not translationally invariant!), to all kind
> of hidden and forgotten approximations, will conspire to make
> any minimization algorithm unusable. I don't know what the
> origin of the above "oscillations" in the energy is: maybe the
> discreteness of the FFT. In any event, I would say that there
> is nothing wrong in the damped dynamics (or in bfgs) itself:
> there is some numerical noise in forces and energy calculation,
> somewhere, God knows where, and whether it can be removed.
>
> By the way: some years ago the energy was printed with 6
> decimals. Increasing the number of decimals to 8 was in
> my opinion a very bad idea: it gives the illusion that those
> last figures are "significant", but in fact they change with the
> phase of the moon.
>
> Paolo
> ---
> Paolo Giannozzi, Dept of Chemistry&Physics&Environment,
> Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
> Phone +39-0432-558216, fax +39-0432-558222
>
>
>
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