Transition State Optimization with Dimer Method

[Natalie Austin]

Hello,

I recently attempted to calculate a transition state for CO2 dissociation 
using CI-NEB. Unfortunately, the NEB did not converge after 66 steps ( It 
ran once up to 32 steps then I had to resubmit it to run another 34 steps) 
and so I stopped the calculation because I noticed that the MAX FORCE for 
the NEB calculation was just fluctuating(see below). Would anyone be able 
to indicate why this is the case? I used the same input for a located a 
transition from physisorbed CO2 to chemisorbed CO2 on the same metal system 
and I did not have this problem. 

I did come across a case in the steps where all criteria was met except for 
the MAX force as shown below: 

 RMS DISPLACEMENT =   0.00013     [ 0.00100]                               
(YES)
 MAX DISPLACEMENT =   0.00225     [ 0.00300]                               
(YES)
 RMS FORCE        =   0.00025     [ 0.00100]                               
(YES)
 MAX FORCE        =   0.00407     [ 0.00300]                               
( NO)

So I took that structure and ran vibrational analysis on it and found that 
it had 3 imaginary frequencies as shown below:

 VIB|                        NORMAL MODES - CARTESIAN DISPLACEMENTS
 VIB|
 VIB|                         1                    2                    3
 VIB|Frequency (cm^-1)  -374.496175          -281.692265           
-60.923863
 VIB|Intensities           0.000000             0.000000             
0.000000
 VIB|Red.Masses (a.u.)    14.024323            16.040216            
53.709296
 VIB|Frc consts (a.u.)    -0.000706            -0.000259            
-0.000002


As a previous user suggested I then tried to use the Transition State Dimer 
method to see if I could guide this structure to a transition state by 
providing the vectors in DIMER _VECTOR for the mode corresponding to the 
state that I was looking for. I was able to locate a potential transition 
state. I've providing the transition state coordinates below. Does it make 
sense to use CI-NEB and reoptimize with the Transition state search in 
cases where it is difficult to converge the NEB?  Is this a common 
occurrence? I'm just trying to make sure that I'm locating transition 
states in an appropriate way. 

Thanks,

Natalie
 
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