[CP2K:10666] K-Points and Extrapolation

[M. Brehm]

Hi Juerg,

thanks a lot!

Best regards,
Martin


Am Donnerstag, 23. August 2018 10:01:54 UTC+2 schrieb jgh:
>
> Hi 
>
> yes, this is an issue that hasn't been looked into since the 
> early implementation of k-points. There is no deep reason why 
> one couldn't use other scheme, but it needs some additional programming. 
> I put it on the todo list. 
>
> regards 
>
> Juerg 
> -------------------------------------------------------------- 
> Juerg Hutter                         Phone : ++41 44 635 4491 
> Institut für Chemie C                FAX   : ++41 44 635 6838 
> Universität Zürich                   E-mail: hut... at chem.uzh.ch 
> <javascript:> 
> Winterthurerstrasse 190 
> CH-8057 Zürich, Switzerland 
> --------------------------------------------------------------- 
>
> -----cp... at googlegroups.com <javascript:> wrote: ----- 
> To: "cp2k" <cp... at googlegroups.com <javascript:>> 
> From: "M. Brehm" 
> Sent by: cp... at googlegroups.com <javascript:> 
> Date: 08/22/2018 01:41PM 
> Subject: [CP2K:10666] K-Points and Extrapolation 
>
>
> Dear community, 
>
> since a few years, CP2k supports K points in DFT calculations, which is a 
> very useful feature. All of the REGTESTs which invlove K points use 
> "EXTRAPOLATION USE_GUESS". For single-point calculations, this is not a 
> problem. However, for geometry optimizations or molecular dynamics runs, 
> this means that the initial guess is performed in every geometry step, and 
> the previous optimized wave function is not at all considered. 
>
> In very simple test cases, it is easy to reach SCF convergence from the 
> initial guess. However, in realistic solid state systems (such as 
> semiconductors), it can be very hard to converge a wave function from the 
> initial guess. This can involve hundreds of SCF cycles, and can even 
> require manual switching of the level of theory (e.g., pre-converge the 
> wave function with PBE, then use this as a starting point for PBE0). If the 
> initial guess is performed in every geometry step, such systems often 
> cannot be treated at all... 
>
> I had a short look in the source code, and saw that two other 
> EXTRAPOLATION methods are allowed in combination with K points, namely 
> "LINEAR_P" and "USE_PREV_RHO_R". However, with the first one, I obtain a 
> SEGFAULT every time in geometry optimization, and with the second one, the 
> SCF convergence is even worse than with using the initial guess. 
>
> So here are my two questions: 
>
> (*) Are these two extrapolation methods supposed to work with K points? 
> Maybe I am just doing something wrong, or there is a small bug which could 
> be fixed. 
>
> (*) If not: Would it be possible to implement a method to just use the 
> last converged wave function for new geometry steps? No sophisticated 
> extrapolation scheme is required. It would be a huge gain if it would be 
> possible to simply copy the last converged wave function. From my (very 
> limited) understanding of the K point implementation in CP2k, this should 
> not be too hard to implement... 
>
> Thank you in advance and with best regards, 
> Martin 
>
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