[CP2K-user] Problem with geometry optimization using LS_SCF

[Torstein Fjermestad]

Dear all, 

First some background for my question:
I got computing hours on a cluster with GPUs. I did some tests with single 
point calculations, and I found that linear scaling SCF ran about 5 times 
faster than "normal" SCF for my system on that cluster. The total energy 
between SCF and LS_SCF differed only by about 0.04 kJ mol-1; and I 
therefore found the results encouraging. 

I then went on to compare LS_SCF and "normal" SCF for geometry 
optimizations on a machine without GPUs. I did three tests:
Test 1: Single point calculation
Test 2: Geometry optimization starting from the geometry in Test 1
Test 3: Geometry optimization starting from the optimized geometry in Test 2

I used the following keywords in the LS_SCF section. 

&LS_SCF
  EPS_FILTER 1.0E-7
  EPS_SCF    1.0E-7
&END

At the beginning of the attached output files the complete input is echoed. 

I now describe the results of the tests.

Test 1: Single point calculation. The total energy differs by 0.05 kJ mol-1

entry

Total energy/ Ha

wall time / s

LS_SCF?

1

-3499.00767

309

yes

2

-3499.00768

380

no


Test 2: Geometry optimization starting from the geometry in Test 1. 
SCF_GUESS ATOMIC is used 

entry

Total energy, first geom / Ha

max. grad of first step

# geometry steps

Total energy, optim geom / Ha

wall time, s

wall time / geom. step, s

LS_SCF?

1

-3499.00767

0.03493

54

Did not converge within the requested wall time

7260 a

134

yes

2

-3499.00769

0.03613

123

-3499.11153

4386

36

no
a Requested wall time in submit script

In Test 2, the wall time per geometry step is much longer when LS_SCF is 
used than when "normal" SCF is used. This is in spite of Test 1 showing 
that LS_SCF is notably faster for a single point calculation. 
Am I doing something wrong here?
Is the start guess of the wavefunction at each geometry step perhaps not 
adequate in the case of LS_SCF?
How should I change the LS_SCF parameters in order to improve the 
calculation?

Test 3: Re-optimization of the optimized structure in Test 2 (entry 2). SCF_GUESS 
ATOMIC is used 
 

entry

Total energy, first geom / Ha

max.grad of first step

# geometry steps

Total energy, optim geom / Ha

wall time

wall time / geom step

LS_SCF?

1

-3499.11150

0.00016

1

-3499.11153

396

396

no

2

-3499.11147

0.00183

19

Did not converge within the requested wall time

7260a

382

yes
a Requested wall time in submit script

Not unexpectedly, in Test 3,  the geometry optimization with "normal" SCF 
converges after 1 geometry step. 
However, the geometry optimization with LS_SCF did not converge within two 
hours and took only 19 geometry steps.
I see that the max. gradient of the first geometry step is much higher in 
the case of LS_SCF than in the case of "normal" SCF. 
Are there some LS_SCF parameters I should adjust to check if the gradient 
has converged?

I would really appreciate help in this matter, because in the current 
situation, I am not able to use LS_SCF for geometry optimizations. 

Thanks and regards,
Torstein Fjermestad

  

 


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