Hello Mohamed,

The way more easy is frozen the distance and watching the trend of energy:

1) Fast optimization for a monomer. This first step will save some steps later.

2)Select monomer, copy-paste in same zone. With alt+may you will translate monomer. With alt+ctrl you will rotate monomer in a line and only with alt you will rote in point. With this keys draw the first structure and with dihedral 1C-5C-13C-9C=180º you will put the monomers in the same axis.

3)Frozen the distances. Here you can frozen distance and for every optimization you done, this one will not change. In this case I would select to freeze the distance 5C-13C. Take care with coma.

4)Now it is necessary to start the search. For example you can start with a distance that makes O----H  3 amst which is a 5C-13C of 5.2 Amst. You opt this structure and write the E(MP2). Then, you can do different steps about 0.4 and 0.2 or 0.1 depending on the value of decreasing of energy  E(3A)>E(2.6A) > E(2.2).... and for example, at the distance of 2.0A you see that the energy is bigger than last, then you find the distance with interaction effects. Take the last structure and then you can apply the TS reaserch.

5)Take care that first "NEGATIVE" vibrations have the inertia to produce the interaction and confirming that is a TS.

It is a short explication (and general) but if you have some question or you are looking for specific method or question, feel free to contact me.

I hope it helps you,

Joaquim Rius

Thanks Joaquim M Rius Bartra

I followed your steps but I got an error message when i tried to optimize the structure of the dimer to calculate the energy :

"error message #2070

The processing of the last link ended abnormally

all processing has been aborted"

Thanks for your help

Can you show the input commands? I recommend that if is your first time, will be more useful do the calculations with dft, fastter and you will see the geometry more or less with the ts. Tomorrow I will try to find a minutes to make an example for you of an two opt.

Hello,

Maybe your error is because you need to add this key  opt=modredun  for read the frozen atoms. If you don't put it, the energy don't converge.

Other think is that it is necessary to start with good symmetry, in this case could be important that distance OH---O  will be the same for both.

Finally point, when you run with structure to TS, you must remove frozen atoms. This is very important.

I add and in/output of an example of opt. The distance is not equal for both OH---H, but with time it is possible to do.

Hi again and thanks for giving your time

here are the input and the output file the distance between OH---O is equal to 3.95 angstrum for both and between C1-C9 is 5.13 angstrum

this structure was obtained with a MP2/6-31Gd optimization of the dimer of acetic acid

With the TS calculation you must add frequency. Then, with the first negative vibration, you must see a vibrational mode between opt and ts. In this case I think that TS vibration should be the H going to one and other O.

With the output, you can do the freq calculation.

Hi Joaquim

so if I understood I should perform a calculation of the frequency first and once the calculation finished I perform another job for the TS

Thanks

The strategy is find a minimum near tS by scanning some structures. Next, when you detect the structure which is high in energy, run a ts search with structure before higher. This ts search it is an optimization without frozen distances. When you have done this opt, the way to know if the structur is correct you shoud do a frequency run with the ts structure that you had obtained. Then, with freq you must see:

1. Only first frequency should be negative.

2. If you put view this negative freq you shoud see that its a part from reactants and the product.

Joaquim Rius

Hi

Iam going to try this and see what it gives

Hi

I got this error message when I run frequency calculation of the acetic acid dimer using B3LYP/6-31G+(d):

CConnection GLOG:Parse_GLOG()

Failure reading vibrational data

line number 524

someone can help

Hello Mohamed,

I found this here in reaserchgate, maybe can helps you.

https://www.researchgate.net/post/What_does_CConnectionGLOGParseGLOG_Gaussian_Error_mean