Hello

I am running neb calculation with lammps for a large system (with 80000 atoms) and I have a convergence issue and I am not sure that I understand well what's going on.

When my convergence criterion is on force (with a ftol of 0.1), the NEB never converges. After 2000 steps of NEB and 2000 steps of CI-NEB the calculation is still not converged. I still have values of the MaxReplicaForce which are usually larger than 1eV/A and can be as large as 20eV/A depending on the system and timestep.

I could of course increase the maximum number of steps but it does not look like MaxReplicaForce will ever converge since it is not really decreasing (for some simulations it's almost constant for some other it changes kind of randomly).

If I got it right MaxReplicaForce will be dependent on the system size : if we have N atoms all having a force of 1eV/A we will have MaxReplicaForce = Sqrt(N) eV/A right? So having 80000 atoms I should maybe have a larger ftol. But I think that even with a small ftol it should converge after some time....

Plus when I check MaxAtomForce it has values which can be larger than 1eV/A which is large. Does this MaxAtomForce includes the spring forces (if not it would be normal to have relatively large values for MaxAtomForce)?

Then when I use a convergence criterion on energy it is converged very quickly. For instance with etol=0.0001eV it is converged in 56 steps. But the energy profile is very different from the one I get after 4000 steps (when I have etol=0).

The energy criterion is met when the maximum energy change of on atom is larger than etol. Right?

Well as you must have now understood, all this is not so clear for me.

Any help is welcome.

Thanks in advance.

Emile

PS : I use lammps1Feb14.

I enclose the output I get with

neb 0. 0.1 2000 2000 100 final FinalImage #: slurm-13024084.out

neb 0.00001 0.1 2000 2000 100 final FinalImage #:slurm-13049634.out

Here is my input file :

slurm-13024084.out (17.6 KB)

slurm-13049634.out (1.95 KB)