[CP2K-user] Poor force convergence wrt basis set size

[Arthur France-Lanord]

Hi, 

I'm currently testing atomic force convergence with respect to basis set 
size, for a crystalline polymorph of sulfur (an array of cyclic S8 
molecules). I've tested both standard GTH basis sets (DZVP to QZV2P 
compared to QZV3P), and MOLOPT (DZVP-MOLOPT-SR to TZV2P-MOLOPT compared to 
TZV2PX-MOLOPT). I'm observing a slow or poor convergence behavior, which 
can be seen in these two plots (MAE : mean absolute error, MAPE : mean 
absolute percentage error, both only computed on the x components of the 
forces): 

GTH basis sets: 
[image: f_basis.png]

MOLOPT basis sets (the TZV2P-MOLOPT/QZV3P panel is just shown here for 
comparison) : 
[image: f_molopt_basis.png]

In particular, TZVP-MOLOPT performs slightly better than TZV2P-MOLOPT..? 
I'm using a high energy cutoff (2500 Ry and a REL_CUTOFF of 100 Ry), and a 
rather tight SCF convergence criterion (1.0e-7) with the OT method. 
Convergence in the SCF is achieved rather fast. 

Are these errors expected? I was trying to reproduce something similar from 
what ccan be seen in https://arxiv.org/pdf/1409.4527.pdf (figures 1 and 2), 
but didn't expect such high relative errors. The configuration I'm using is 
from a molecular dynamics run to obtain non-zero forces. I'm including a 
cp2k input file to reproduce the QZV3P calculation. 

many thanks, 
Arthur

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