[CP2K-user] massive slowdown in SCF timings during MD

[Sean Fischer]

I'm attempting to run MD using QS on a 64 molecule water box. So far I have 
been unable to achieve consistent performance across the MD trajectory. 
After a while the time per MD step dramatically increases, but the number 
of SCF iterations per step stays basically constant, as you can see in the 
included image.

[image: md_timing.png] <about:invalid#zClosurez>

To further illustrate this, the following is from one of the first steps in 
the trajectory


  ----------------------------------- OT 
---------------------------------------

  Minimizer      : DIIS                : direct inversion

                                         in the iterative subspace

                                         using   7 DIIS vectors

                                         safer DIIS on

  Preconditioner : FULL_ALL            : diagonalization, state selective

  Precond_solver : DEFAULT

  stepsize       :    0.15000000                  energy_gap     :    
0.08000000

  eps_taylor     :   0.10000E-15                  max_taylor     :         
    4

  ----------------------------------- OT 
---------------------------------------


  Step     Update method      Time    Convergence         Total energy    
Change

  
------------------------------------------------------------------------------

     1 OT DIIS     0.15E+00    2.3     0.00030148     -1107.9047914512 
-1.11E+03

     2 OT DIIS     0.15E+00    1.6     0.00012826     -1107.9089080136 
-4.12E-03

     3 OT DIIS     0.15E+00    1.6     0.00007994     -1107.9095257164 
-6.18E-04

     4 OT DIIS     0.15E+00    1.6     0.00000744     -1107.9096948950 
-1.69E-04

     5 OT DIIS     0.15E+00    2.0     0.00000377     -1107.9096964458 
-1.55E-06

     6 OT DIIS     0.15E+00    1.6     0.00000067     -1107.9096967653 
-3.19E-07


  *** SCF run converged in     6 steps ***



  Electronic density on regular grids:       -511.9999993695        
0.0000006305

  Core density on regular grids:              511.9999999846       
-0.0000000154

  Total charge density on r-space grids:        0.0000006151

  Total charge density g-space grids:           0.0000006151



While after ~2500 steps, the same section now looks like


  ----------------------------------- OT 
---------------------------------------

  Minimizer      : DIIS                : direct inversion

                                         in the iterative subspace

                                         using   7 DIIS vectors

                                         safer DIIS on

  Preconditioner : FULL_ALL            : diagonalization, state selective

  Precond_solver : DEFAULT

  stepsize       :    0.15000000                  energy_gap     :    
0.08000000

  eps_taylor     :   0.10000E-15                  max_taylor     :         
    4

  ----------------------------------- OT 
---------------------------------------


  Step     Update method      Time    Convergence         Total energy    
Change

  
------------------------------------------------------------------------------

     1 OT DIIS     0.15E+00    6.3     0.00003184     -1108.0316770158 
-1.11E+03

     2 OT DIIS     0.15E+00   17.0     0.00001599     -1108.0317180830 
-4.11E-05

     3 OT DIIS     0.15E+00   18.4     0.00000956     -1108.0317247395 
-6.66E-06

     4 OT DIIS     0.15E+00   21.9     0.00000119     -1108.0317273970 
-2.66E-06

     5 OT DIIS     0.15E+00    6.8     0.00000063     -1108.0317274283 
-3.14E-08


  *** SCF run converged in     5 steps ***



  Electronic density on regular grids:       -511.9999994472        
0.0000005528

  Core density on regular grids:              511.9999999845       
-0.0000000155

  Total charge density on r-space grids:        0.0000005373

  Total charge density g-space grids:           0.0000005373



This has happened with different functionals, pseudopotentials, and SCF 
parameters (PS vs ASPC extrapolation, different orders of extrapolation, OT 
DIIS vs DIIS/Diag., different preconditioners). I recompiled the whole code 
with the toolchain install script generating all dependencies and got the 
same result. There is no indication from the dynamics of the system itself 
as to what the problem might be as the potential and kinetic energies are 
well behaved, and the constant of motion for the dynamics is reasonably 
conserved. I've watched the resulting trajectory itself, and there is 
nothing unusually happening (no bonding breaking, etc.) The input file I 
used for run in the plot above is attached. The starting geometry for the 
water molecules was taken from the 64 molecule benchmark in the tests/QS 
directory.


If anyone has any suggestions on possible causes or where I can look for 
more information, I would be very appreciative.
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