[Pw_forum] What to do when NEB calculation is difficult to converge?

[Xunlei Ding]

Dear all,
I am now doing a neb calculation on a surface adsorption system with 
pwscf3.0
The default values are used in the calculation as below:
     calculation                   =  neb
     restart_mode                  =  restart
     opt_scheme                    =  quick-min
     num_of_images                 =  7
     nstep                         =  500
     first_last_opt                =  F
     coarse-grained phase-space    =  F
     use_freezing                  =  F
     ds                            =  1.0000 a.u.
     CI_scheme                     =  highest-TS
     k_max                         =  0.6169 a.u.
     k_min                         =  0.6169 a.u.
     path_thr                      =  0.1000 eV / A
     initial path length           = 12.2601 bohr
     initial inter-image distance  =  2.0434 bohr

It has more than 400 iterations and has not converged. Should I change 
some settings, such as ds, or k_max and k_min?
Below are the activation energies for the last few iterations, and they 
are not monotone decreasing but oscillating.
Would anybody give me some suggestions?

     activation energy (->) =   0.273443 eV
     activation energy (->) =   0.273431 eV
     activation energy (->) =   0.273719 eV
     activation energy (->) =   0.273765 eV
     activation energy (->) =   0.273207 eV
     activation energy (->) =   0.273148 eV
     activation energy (->) =   0.273311 eV
     activation energy (->) =   0.273501 eV
     activation energy (->) =   0.272957 eV
     activation energy (->) =   0.272818 eV
     activation energy (->) =   0.272865 eV
     activation energy (->) =   0.273155 eV
     activation energy (->) =   0.272750 eV
     activation energy (->) =   0.272463 eV
     activation energy (->) =   0.272443 eV
     activation energy (->) =   0.272777 eV
     activation energy (->) =   0.272574 eV
     activation energy (->) =   0.272118 eV
     activation energy (->) =   0.272080 eV
     activation energy (->) =   0.272367 eV
     activation energy (->) =   0.273228 eV
     activation energy (->) =   0.272663 eV
     activation energy (->) =   0.272621 eV
     activation energy (->) =   0.272935 eV
     activation energy (->) =   0.273033 eV
     activation energy (->) =   0.272427 eV
     activation energy (->) =   0.272386 eV
     activation energy (->) =   0.272558 eV
     activation energy (->) =   0.272823 eV
     activation energy (->) =   0.272391 eV

Yours,
Ding