[CP2K:7678] Transition state searches with Dimer method

Jörg Saßmannshausen j.sassma... at ucl.ac.uk
Mon Apr 18 15:48:20 CEST 2016


Hi Tobias,

jumping into this thread here: I got similar problems like you. I am using the 
SM Band type and I get quite a number of imaginary frequencies next to the one 
imaginary frequency for the transition state when I am doing a frequency 
analysis of the suggested TS. Thus, like you I want to obtain a better result 
and get rid off these spurious imaginary frequencies. I had a look at them and 
most of them appear to be movements of an atom within the caged framework of 
my compound.

So there seems to be a more common problem with NEB here. Unless we both do it 
consistantly wrong it does produce unwanted imaginary frequencies.

As you are using the dimer methode: Can you actually read in the hessian of 
the TS so the program has a good starting point to find t he true TS? I know 
that is the common practice in other DFT codes.

Also: could somebody point me in the direction of how to set up a dimer 
calculation properly? Maybe that would solve Tobias and my problem.

All the best from a cold but dry London

Jörg

On Donnerstag 14 April 2016 Tobias Kraemer wrote:
> Hi all,
> 
> 
> I would like to come back to a question regarding the dimer method for TS
> searches. I have located a number of TS by the CI-NEB method, comfirmed by
> analysis of their harmonic frequencies. Some contain spurious imaginary
> modes, which I would quite like
> to get rid of, some are well converged to a 'real' TS. Either way, I want
> to re-optimize these TS with the dimer method, to somewhat tighter
> geometrical convergence criteria (for reasons I have discussed on this
> forum before, I had to use somewhat sloppy thresholds
> for SCF and geometries in the CI-NEB calculations, but the obtained TS are
> 'good guesses' nonetheless). In a number of cases this was quite
> successful, leading to TS as expected, even thought the dimer method would
> take quite a large number of steps. I am using
> vectors corresponding to the reaction path obtained from the frequency
> analysis by the way, in order to guide the calculation to the true
> transition state structure. However, I am having some significant trouble
> locating some of these TS (even though the NEB energy profile
> looks very reasonable, and the imaginary vibrational frequencies modes look
> ok). Even after 1000 steps, there is no convergence in sight, and in some
> cases (by visual inspection) the geometry completely escapes the TS region
> and goes back to what appears to be the ground state. I am wondering if
> there is anything I can do about the parameters for the dimer method, to
> (a) prevent this from happening and (b) get faster convergence. These
> calcualtions take ages to be honest, and sometimes don't yield the desired
> result even. Continuous restarts and hoping for the best are not the
> solution I feel. See below the relevant section from my input. Any
> suggestions are welcome.
> 
> 
> 
> &MOTION
>   &GEO_OPT
>     TYPE TRANSITION_STATE
>      &TRANSITION_STATE
>        METHOD DIMER
>         &DIMER
>          DR [angstrom] 0.01
> !         ANGLE_TOLERANCE 1.5
>          INTERPOLATE_GRADIENT T
>          &ROT_OPT
>           OPTIMIZER CG
>           MAX_ITER 200
>           &CG
>           MAX_STEEP_STEPS 0
>           &LINE_SEARCH
>             TYPE 2PNT
>            &END LINE_SEARCH
>           &END CG
>          &END ROT_OPT
>          &DIMER_VECTOR
>           @INCLUDE 'vector'
>          &END DIMER_VECTOR
>         &END DIMER
>      &END TRANSITION_STATE
>     OPTIMIZER CG
>       &CG
>        &LINE_SEARCH
>          TYPE 2PNT
>        &END LINE_SEARCH
>       &END CG
>     MAX_FORCE 1.0E-4
>     MAX_ITER 1000
>   &END GEO_OPT
>   &PRINT
>      &TRAJECTORY
>       &EACH
>          GEO_OPT 1
>       &END EACH
>     &END TRAJECTORY
>   &END
> &END MOTION
> 
> 
> Tobias


-- 
*************************************************************
Dr. Jörg Saßmannshausen, MRSC
University College London
Department of Chemistry
20 Gordon Street
London
WC1H 0AJ 

email: j.sassma... at ucl.ac.uk
web: http://sassy.formativ.net

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