Hello,
Does anybody know if there is a different convergence criterion when using LBFGS as the optimizer in a dimer calculation?
The reason I ask is as follows:
I have a calculation where I run a preliminary calculation with lowered plane wave kinetic energy, SCF tolerance and kpoint sampling using the FIRE optimizer to get an initial guess to be used in a more stringent calculation. With this initial run, I ensure that the curvature is consistently negative and the force on the dimer drops until it is lowish.
Now using the geometry and unit vector along the dimer axis from the preliminary calculation, I increased the cutoff, SCF tolerance (EDIFF = 1e7) and k sampling (note: these parameters give wellconverged energies for stable state calculations) whilst switching to the LBFGS optimizer (with default settings). This new calculation converged with all forces on nonfixed atoms dropping below my desired accuracy of EDIFFG = 0.001. However, the dimer force does not converge to less than this  in fact it appears to be over an order of magnitude greater (see below) despite the calculation stopping due to reaching the convergence criteria...
Step Force Torque Energy Curvature Angle
1 0.04872 26.20388 89.70996 13.89656 30.02478
1 0.04872 7.50580 89.70996 1.21436 16.09547
1 0.04872 6.12038 89.70996 3.16422 7.89352
1 0.04872 2.91902 89.70996 3.98803 5.69917
2 0.04872  89.70996  
When using the dimer in the past I have always used the FIRE or QM optimizers and this is the first time I've used LBFGS  FIRE and QM, in my experience always converge to Forces on dimer < EDIFFG. Does this optimizer have different convergence criteria? Why does the dimer force not converge with the forces on the atoms?
Thanks, any help or suggestions are very much appreciated :)
LBFGS Convergence Criterion
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Re: LBFGS Convergence Criterion
If you can make the files for these calculations available (both the FIRE calculation and then the subsequent LBFGS calculation), I can then take a closer look at what's going wrong.
Also, just to make sure I understand, you are saying that the dimer force printed (0.04872) is significantly higher than your convergence criterion (0.001) and yet it converged after the first step  does that sum it up?
Also, just to make sure I understand, you are saying that the dimer force printed (0.04872) is significantly higher than your convergence criterion (0.001) and yet it converged after the first step  does that sum it up?
Re: LBFGS Convergence Criterion
Sure, I've attached them to this post  though checking the calculation I used the CG instead of FIRE for the preliminary calculation. The issue with LBFGS is the same though.
Yeah, that's right  the forces on each freetomove atom in the OUTCAR have converged to be lower than EDIFFG (0.001), however the dimer force as reported in the DIMCAR is much higher (0.04872).
Thanks for taking a look.
Yeah, that's right  the forces on each freetomove atom in the OUTCAR have converged to be lower than EDIFFG (0.001), however the dimer force as reported in the DIMCAR is much higher (0.04872).
Thanks for taking a look.
 Attachments

 Dimer_troubleshooting.zip
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Re: LBFGS Convergence Criterion
BFGS is a pretty reliable method
https://www.everipedia.com/Limitedmemory_BFGS/
https://www.everipedia.com/Limitedmemory_BFGS/