Kpoints, Diagonalization algorithm and Mixing

[Lucas Lodeiro]

Hello all,

I am trying to improve the performance of MD steps for a big slab system which contains Cu atoms... the systems has ~1500 electrons and needs 2x2x1 kpoints to compute their electronic structure (as we explore in a PW based program, Quantum Espresso).

The first approach was to use multiple_cell 2 2 1 with just a Gamma point and &OT method (with FULL_KINETIC preconditioner). The system is well behaved and works fine, but the time needed for each frame is a little high (87s per frame) to compute long MD.

As a second approach, to explore if there are a method to speed up the MD, we use multiple_cell 1 1 1 and 2x2x1 kpoints, with standart diagonalization algorithm with broyden mixing (attached file), but the performance is not so good, mainly in the extrapolation step, where ASPC cannot be used. We explore LINEAR_P, and previous P and Rho, but this does not work fine... and needs several SCF steps to converge.

The combinations for Diagonalization algorithm and mixing are various and I prefer to ask if there is a way to improve diagonalization based calculations.

As the system is well behaved, is it an advantage to use another algorithm and mixing method?

Could be a diagonalization-kpoint calculation faster than the analogous one with OT?

Another thing is based on the kpoint scheme... We want to use a 2x2x1 gamma centered grid... but I am not clear about MONKHORST-PACK is the Gamma centered or displaced grid. MACDONALD and GENERAL, which are their differences? I am familiar with the Quantum-Espresso kpoint scheme.

There are a lot of questions, sorry, but I need some insight to not search in the fog, one by one.

Regards - Lucas Lodeiro

[Marcella Iannuzzi]

Dear Lucas, 

With OT you can try other preconditioners.

The best would be FULL_ALL, but is also the slowest. FULL_SINGLE_INVERSE should be a good compromise. 

With the diagonalisation the extrapolation is probably better using the previous wavefunction. 

To speed up MD you can also consider to make the convergence criteria less tight, but keep an eye on the conservation of the total energy.

Kind regards

Marcella

[Lucas Lodeiro]

Dear Marcella,

As you mention, with OT method I tried with different preconditioners. FULL_KINETIC is 20% faster than FULL_SINGLE_INVERSE, the other preconditioners are slower. Thanks for the advice, I taked it into account, thinking in order to reduce the CUTOFF and/or increase the EPS_SCF (Which another could be a useful change to speed up?), but I preferred to search another method first, to maintain the calculation tightness.

In the case of Diagonalization method with Kpoints, the extrapolation methods based on wavefunction are not available... only LINEAR_P, USE_PREV_P and USE_PREV_RHO, and their performance is not very good for an MD calculation. :(

Regards

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