CUTTOFF and REL_CUTTOFF for large molecular system

[Tobias Kraemer]

Dear CP2K group,


being a relatively new cp2k user, I was wondering if this group could give 
me some advice on the choice of (REL) CUTOFF values.
The system I am considering is a somewhat extended molecular crystal 
structure, containing organometallic fragments (basically a rhodium 
phosphine complex with additional ligand plus large borate counteranions 
(BArF4-).The full system contains about 600+ atoms in the unit cell, 
basically 4 cation/anion pairs 
(asymmetric units). I think the system cannot be reduced in size, and the 
full unit cell has to be taken into account.I have attached a cif file 
which gives you
an idea about the kind of system (not exactly the one used in the 
calculation below, but very much related)
 
The basic idea behind determining the CUTOFF is clear to me and I have done 
the tutorial on bulk Si. I have used the same protocol for my system 
(running a series of calculations with varying CUTOFF and REL_CUTOFF 
values). In contrast to the suggested method in the tutorial, I have 
fully converged the SCF cycle) Below are the (first) results of this 
series. The energy appears to be converging towards higher CUTOFF values 
(some more calculations are currently running)

(1) I wonder if choosing such a large value for the cutoff is reasonable? 
It should also increase the accuracy of the calculations, but also the cost.
Also, with respect to the number of Gaussian being mapped onto the finest 
grid, what would one need to look for?  
In the tutorial the decision was based on the fact that "the distribution 
of Gaussian functions on the grids are reasonable". Could somebody 
elaborate on this 
and give me some guidance how to decide this?
(2) How does the size of the CUTOFF value correlate with the system size, 
if there is any correlation?  
(3) Are there some rule of thumbs as how to choose the REL_CUTOFF with 
respect to the CUTOFF? For now I have tried 30, 60 and 100 Ry.
But I may need to go to higher numbers in this particular case. 



# Grid cutoff vs total energy
# Date: Tue Jun 10 17:26:55 BST 2014
# REL_CUTOFF = 60
# Cutoff (Ry) | Total Energy (Ha) | Charge | SCF | | NG on grid 1 | NG on 
grid 2 | NG on grid 3 | NG on grid 4
     50.00  -4583.1402739393   -8.4046446120  200  1623260  248362    
2112       0
    100.00  -4477.8479306771   -0.0745436930  128  1264867  519816   
89051       0
    150.00  -4479.5978060995   -0.0191170935  128  1150415  472845  
248362    2112
    200.00  -4478.5030451759    0.0008175249  128  905641  531280  
432781    4032
    250.00  -4478.4529157377   -0.0000917804  128  838757  500556  523109   
11312
    300.00  -4478.4141842105   -0.0000534995  128  796882  467985  519816   
89051
    350.00  -4478.3827319182   -0.0000170792  128  748308  474790  454036  
196600
    400.00  -4478.3597765363    0.0000003328  128  679828  499675  489931  
204300
    450.00  -4478.3498795935    0.0000000149  128  621687  528728  472845  
250474
    500.00  -4478.3399690553    0.0000000147  128  567997  416013  600098  
289626
    550.00  -4478.3338555119   -0.0000000007  128  485909  446314  559882  
381629
    600.00  -4478.3312330111    0.0000000007  128  442629  463012  531280  
436813
    650.00  -4478.3291405214    0.0000000006  128  399089  473084  554024  
447537
    700.00  -4478.3308119206    0.0000000008  128  375655  473218  509402  
515459
    750.00  -4478.3298871109    0.0000000008  128  365399  473358  500556  
534421
    800.00  -4478.3266993684    0.0000000007  128  353459  468098  461722  
590455
    850.00  -4478.3266987930    0.0000000007  128  348184  458373  472494  
594683
    900.00  -4478.3273848891    0.0000000007  128  336984  459898  467985  
608867
    950.00  -4478.3277135782    0.0000000007  128  309758  476872  469877  
617227
   1000.00  -4478.3277705414    0.0000000008  128  291089  482269  455857  
644519
   1050.00  -4478.3278605666    0.0000000007  128  229036  519272  474790  
650636

Some further questions;

(4) Could you recommend some literature (books) which explains the theory 
of solid state calculations, k-sampling, planewaves?

(5) As a next step I would like to manipulate the structure, i.e. 
rotate/move ligands around? Is there a good visualizer that can do such 
things for periodic systems? The problem I am facing is that I would like 
to do these things simultaneously for all 4 asymmetric units.
I think it becomes impracticable to do this for each unit separately. I 
have started using GDIS but I am a little bit uncertain about its
abilities (great program though!!). Has anyone experience with VESTA? I am 
particularly looking for free software. 


Thank you very much for your help, this is very much appreciated. There are 
a lot of questions right a the beginning, and it is not always easy to 
find some answers.   

Best


Tobias


Dr. Tobias Kraemer

Research Associate

Institute of Chemical Sciences

School of Engineering & Physical Sciences

Heriot-Watt University 

Edinburgh EH14 4AS

United Kingdom

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