# cutoff energy convergence problem with a brucite slab

Eszter Makkos eszter... at gmail.com
Mon Aug 3 14:14:42 UTC 2015

```Hi,

I would like to optimise a brucite surface model with CP2K (and continue
further with adsorption studies). I am a new user of CP2K, but I tried to
follow the tutorials to create an input and run a SCF calculation with it.
Brucite (Mg(OH)2) has a layered crystal structure with a hexagonal lattice
cell. I made a two layered slab with 90 atoms in total for start and I used
the option PERIODIC XYZ for the cell and for the Poisson calculations.
Although a simple SCF calculation converged pretty easily in 22 steps (with
a cut_off energy 400 Ry and rel_cutoff 60 Ry) when I try to converge the
cutoff energies, I run into problems. The Total energy does not converge,
it is not even close to the 10E-08 energy difference which was suggested in
the tutorial (http://www.cp2k.org/howto:converging_cutoff):

# Cutoff (Ry) | Total Energy (Ha) | NG on grid 1 | NG on grid 2 | NG on
grid 3 | NG on grid 4 | NG on grid 5
100.00      -615.4400995077     169106           83084            32296
150                 0
150.00      -613.5889140772     133746           79710            70940
240                 0
200.00      -613.7087067681     106708           75442            97262
5224               0
250.00      -613.7093656033     103150           68644            92846
19996              0
300.00      -613.7092929853     83074             86032
83084           32296             150
350.00      -613.7094737862     75612             77286
99292           32296             150
400.00      -613.7094938770     69804             67896
83292           63404             240
450.00      -613.7095035985     68764             64982
79710           70940             240
500.00      -613.7095069116     58396             57658
89650           78692             240
550.00      -613.7095524421     53260             56148
72742          102246            240
600.00      -613.7095832788     51740             54968
75442           97262            5224
700.00      -613.7094980637     50068             53572
68154           92846            19996
800.00      -613.7095566028     33992             57114
78150           83084            32296

I attached the input file to my email, it would be great if someone had a
few suggestions what could I try to get it converge.

Best wishes,
Eszter

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