I am trying to get the IRC path for my molecule. I get the IRC curve, which is not the expected one (the curve looks like stair case structure). Even though I am getting the optimized structure, the output file is the same as that of input file.
Can any one suggest how to get the correct IRC?
Input file is like this......
%nprocshared=8
%mem=2GW
# irc=(rcfc,maxcyc=200) b3lyp/6-31g(d) nosymm scf=(qc,maxcyc=200) geom=connectivity
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coordinates
Thanks in advance
Chethan
Sorry but I did not understand the problem. The input file looks ok. I guess you are using the transition state as input geometry, right? Because the calculation will use it as starting point to follow the gradients toward reactants in one side and to products in the other side of the IRC. If the calculation works fine, you should get a number of optimized structures and a list of energy points along the reaction coordinate. I have attached an example output file. In this case, the calculation died because it exceeded the number of iterations in one of the last steps, but I think it can help you.
Sorry but I did not understand the problem. The input file looks ok. I guess you are using the transition state as input geometry, right? Because the calculation will use it as starting point to follow the gradients toward reactants in one side and to products in the other side of the IRC. If the calculation works fine, you should get a number of optimized structures and a list of energy points along the reaction coordinate. I have attached an example output file. In this case, the calculation died because it exceeded the number of iterations in one of the last steps, but I think it can help you.
I think first you freshly draw the molecules optimise it using optfreq, then run for QST2 instead of B3LYP then try to adjust bond distance and length, save another file then go to IRC ........ Dont draw same molecules, numbering should be same...........
Best of luck
Dear Yogesh
Thank you
I draw the new structure and optimized with freq. And this structure is good to get the TS (QST3 - with guess structure). But if i used the same initial input structure am unable to get the IRC (file is different but the structure numbering position same).
I am wondering the output file is same as input file (reactant). According to you B3LYP is not necessary?
Is it so?............... I will check it again........ long time i have not done any IRC....... I will do first them reach to you
The keyword B3LYP refers to the calculation level, while QST2 refers to the type of calculation. Therefore both are necesary. Chethan, did you check the output file I uploaded?
Dear Chethan
I believe you used a non-optimized TS structure for your IRC calculations. So i recommend you to perform QST2 or QST3 calculations in order to find a fine TS structure and approve it by freq calculations and then do IRCs.
start by confirming your TS by frequency calculation and then you do irc
It is necessary to specify the check point file of the calculation where you have obtained the imaginary frequency i.e is the transition state. Otherwise you may not get the desired result.
I will suggest you use spartan calculation method. It makes it much easier and you can plot the curve easilty
Please please and please use NEB calculation instead of IRC. NEB is very powerful method to find a minimum reaction pathway that is not comparable with IRC. NEB is implemented in NWChem and quantum espresso as free codes.
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