Some new keywords

Force

# force ub3lyp/cc-pvdz geom=connectivity

This calculation type keyword requests a single calculation of the forces on the nuclei (i.e. the gradient of the energy).
And it appears in the output as follows :

         Item               Value     Threshold  Converged?
 Maximum Force            0.021620     0.000450     NO 
 RMS     Force            0.008935     0.000300     NO 
 Maximum Displacement     0.172981     0.001800     NO 
 RMS     Displacement     0.056998     0.001200     NO 
 Predicted change in Energy=-7.648262D-03

For more explanations : Force

Stable

# stable=opt ub3lyp/cc-pvdz geom=connectivity

This calculation type method requests that the stability of the Hartree-Fock or DFT wavefunction be tested.
And it appears in the output as follows :

The wavefunction has an RHF -> UHF instability.

Or in the case of a stability :

The wavefunction is stable under the perturbations considered.

If you get an instability you can ask Gaussian to re-optimize your wavefunction, you just have to put stable=opt in your keyword. With this one, Gaussian will test the stability of your wavefunction and in the case of an instability it will re-optimize it.

More explanations on Gaussian website : Stable

Guess

# guess=read force ub3lyp/cc-pvdz geom=connectivity

This keyword controls the initial guess for the Hartree-Fock wavefunction. Guess is not meaningful without an option. So in our case we will use guess=read and we will specify the force keyword (because we would like that Gaussian uses the .chk of our stable calculation, read the geometry and calculate the force).

We will use the keyword guess=read freq too, and so it will give us the frequencies and the force at the end. Thus we will see if we got a transition state, and because in the article they noted the different frequencies of their molecules, we will know if we have really find the same TS, or another one most often higher in energy...

More explanations on Guess

NoEigenTest

Sometimes, Gaussian does not succeed optimize the geometry because it find less or more imaginary frequencies as expected, so you will get some error message like this :

Optimization stopped.
    -- Wrong number of Negative eigenvalues: Desired=  1 Actual=  2
    -- Flag reset to prevent archiving.

And so to prevenet this sort of results, you can put the NoEigenTest in the input.

# opt=(rcfc,ts,noeigentest) freq ub3lyp/cc-pvdz geom=connectivity

This keyword, NoEigenTest, suppresses testing the curvature in Berny optimizations, and so forces Gaussian to optimize.

More explanations on EigenTest

MaxStep

#p opt=(maxstep=2) guess=read freq ub3lyp/cc-pvdz nosymm

To set the maximum size for an optimization step. So you can fix small step when you do your optimization.

More explanations on Maxstep

MaxCycle

#p opt=(maxstep=2,rcfc,maxcycle=10) guess=read freq geom=check ub3lyp/cc-pvdz nosymm

To set the maximum number of optimization steps.

More explanations on Maxcycle

Step

 #p guess=read freq geom=(check,step=6) ub3lyp/cc-pvdz nosymm

To consider a specific step in our optimization and ask for some calculation concerning this particular step.

More explanations on Step

Next part : Transition state TS1

Back to Get back the results found in an article

Back to Suggestion of a transition state between propene and cyclopropane