Transition state searches with Dimer method

Tobias Kraemer 161brun... at
Thu Apr 14 11:20:49 UTC 2016

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.

         DR [angstrom] 0.01
!         ANGLE_TOLERANCE 1.5
          OPTIMIZER CG
          MAX_ITER 200
          MAX_STEEP_STEPS 0
            TYPE 2PNT
           &END LINE_SEARCH
          &END CG
         &END ROT_OPT
          @INCLUDE 'vector'
        &END DIMER
         TYPE 2PNT
      &END CG
    MAX_FORCE 1.0E-4
    MAX_ITER 1000
         GEO_OPT 1
      &END EACH

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