Currently, I have several transition state structures for a reaction of the kind A + B -> AB. These TS structures were calculated using Gaussian's own Berny-Optimization procedure and validated as first-order saddle-points by performing frequency calculations. Each of the structures has one, and only one, negative frequency. (It really doesn't matter for this question, but these are DFT calculations and they don't take too long as the system is moderatly small).
To validate and analyze these transition-states in the realm of the bigger picture of the whole PES some more, I would like to perform optimizations starting from them to yield the two minima connected by the transition-state.
Using the Gaussian software the most primitive idea to do this is to perform a regular optimization. I of course did this and, oddly, for every transition state the structure of the product of the reaction is yielded. Conceptually, a true transition-state on the PES should have zero gradient. This means that starting a primitive, gradient-based optimization from a hypothetical, 100% converged transition state should not yield another minimum anyway. I assume that the Gaussian software is in this case too "smart" and "user-friendly": The optimization might not be performed along the imaginary mode or using only the gradients, but most likely all convergence-accelearting black magic available is used. For most cases, this is a reasonable default scenario as it speeds up the optimization towards a truely relevant structure. However, I am also interested in the "path" of the optimization downward the imaginary mode. So this does not work for me.
It seems like the "EigenvectorFollowing" (EF) option for Opt in Gaussian is exactly what is needed to solve my problem. However, I cannot seem to get it to work for a regular optimization starting from the TS structure. Keywords that have failed include:
#p opt(z-matrix,EF) freq b3lyp/aug-cc-pVDZ
#p opt(cartesian,EF) freq b3lyp/aug-cc-pVDZ
#p opt(EF) freq b3lyp/aug-cc-pVDZ
#p opt(calcall,EF) freq b3lyp/aug-cc-pVDZ
#p opt(calcfc,EF) freq b3lyp/aug-cc-pVDZ
#p opt(readfc,EF) freq b3lyp/aug-cc-pVDZ
In every case, the error is the same:
(Enter /apps/gaussian/g09_D/g09/l113.exe)
EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-EF-
EIGENVECTOR FOLLOWING MINIMUM SEARCH
INITIALIZATION PASS
************************************************
** ERROR IN INITEF. NUMBER OF VARIABLES ( 0) **
** INCORRECT (SHOULD BE BETWEEN 1 AND 50) **
************************************************
Searching for this error on the internet yields many results, however they are all realted to MP4 / CCSD(T) optimizations and not to my scenario. I have also tried to specify the molecule as Z-Matrix or Cartesian coordinates.
Note that - speaking plainly - the system in concideration is two organic molecules and previous investigations showed that it is quite boring from a quantum-chemical viewpoint. By this I mean that the structures are "well-behaved" and not biradicals or other quantum-chemical-corner-cases where DFT fails.
I hope that this issue can be resolved by an experienced gaussian user. I am using Gaussian 09 Rev. E. The first part of my question therefore is how can I use the EF option of Gaussian to perform an optimization along an imaginary mode starting from a transition structure?
Secondly, I still do not have any idea how to reach the "second" minimum connected by the TS (assuming the reaction is concerted, this would be the seperated educts). The only possible approach to tackle this which I can think of currently is to perturb the system manually along the imaginary mode and perform EigenVector-Following optimizations from these perturbed structres. Is there a more elegant way, circumventing manual structural modifications, to yield the two minima connected by the TS?
