[Pw_forum] Manual Kpoint Optimization for a Monoclinic lattice structured organic compound

[Giovanni Cantele]

1) what kind of convergence you haven’t reached yet? a) convergence of structural optimisation (atomic positions), or b) convergence of unit cell optimisation (e.g. if you use calculation=‘vc-relax’), or c) self-consistency cycle

2) if by improving the sampling of the Brillouin zone (2x2x2, 3x3x3, ….) nothing happens, maybe it can be that the parameter that prevents you system from reaching convergence is not the k-point grid!!! Which pseudo potentials do you use, and with which cutoff? For organic crystals, van der Waals interactions might be important as well, have you already tried to switch them on? Are you sure that the input positions are right, for example by visualising your input file with XCrysden (such a practice is advisable BEFORE any, maybe computationally expensive, run is started!)

3) the “acceptable" size of the k-point grid depends on the system under investigation and on the property you are interested in (for a given grid, there are properties that might be converged, and others that might be not!). 

4) concerning grids that look like n x m x l instead of n x n x n , usually if for example axes a and b are in the ratio a/b = u, than n and m have the inverse ratio (n/m ~ 1/u). For example: a = b = c, use 4 x 4 x 4 ; a = b, c = 2a, use 4 x 4 x 2 (THIS ARE JUST EXAMPLES, THAT DO NOT PROVIDE CONVERGENCE CRITERIA!).

GIOVANNI



> On 04 Dec 2015, at 14:23, Ridwan Agbaoye <agbaoyeridwan at yahoo.com> wrote:
> 
> 
> 
> On Thursday, December 3, 2015 11:41 AM, Ridwan Agbaoye <agbaoyeridwan at yahoo.com> wrote:
> 
> 
> I am doing structural optimization for a monoclinic latice structured organic compound with 13 atoms, 4 types of atom (C,H,O and S) and  , i need expertise advice on how my automatic kpoint grid required for optimization will look like, i tried 2 2 2 1 1 1, 3 3 3 1 1 1, 4 4 4 1 1 1... 9 9 9 1 1 1 but the result havent reached convergence yet even with the very very high computation time it require 
> 
> Recently i learnt i am not suppose to use the uniform kpoint grid but the ones that looked like 6 4 6 1 1 1 , 6 1 1 1 1 1, 8 4 8 1 1 1 etc
> 
> please help me with the best automatic kpoint grid required for my system and how to determine the kind of kpoint grid to use for subsequent system calculation 
> 
> Thank you very much
> 
> 
> Agbaoye Ridwan Olamide
> M.Sc. (in view)
> Federal University of Agriculture, Abeokuta
> 
> 
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