<div dir="ltr">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 </div>