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IRC Calculations

3 IRC Calculations
Cristina Nevado Feb. 15, 2020, 2:11 a.m. #96

Good afternoon,

I am working with GRRM17 to run IRC from transition states calculated in Gaussian.

When I use Gaussian to calculate IRC, I run 2 jobs, “reverse” and “forward”. Then, I can check that with “reverse” log file  I get the previous structure of the transition state and with “forward” file I can see the movement from the transition state to the product. When I run the IRC calculation in GRRM, I can see only the forward movement (from the transition state to the product).

Is it possible also to check the reverse movement from transition state to previous stratucture (or stating structure)?

Thank you very much

Best regards

 

 

  • Re: IRC Calculations
    Kenichiro Saita

    What options do you specify in your GRRM17 input file?

    In GRRM17, the IRC calculation always computes forward and backward IRC paths (unless the "Meta-IRC” option is specified).

    Of course, the forward and the backword paths can be computed from a saddle point (transition state). In other words, from a non-stationary point, only a mass-weighted steepest-descent path can be computed.

    Did you see the string "IRC FOLLOWING (FORWARD) STARTING FROM FIRST-ORDER SADDLE” in your GRRM17 log file? If you got "STEEPEST-DESCENT PATH FOLLOWING STARTING FROM NON-STATIONARY POINT”, it meant your input structure was not recognized as a saddle point.

    Even if your input structure was recognized as a 1st-order saddle point, it might correspond to the transition state structure between two conformers of the product.

    When you use the pre-optimized transition state structure by Gaussian (without GRRM17), such a problem sometimes appears. The SCF cycles might be converged in a different electronic structure. Try using the “MO Guess = filename.chk” option in order to read the Gaussian Checkpoint file as a proper initial guess.

    Or try the SADDLE calculation before the IRC calculation. The IRC paths can be automatically computed by specifying “Saddle+IRC” option.

    The DS-AFIR calculation would be able to help you to get the IRC paths between the reactant and the product. (Like the QST2 method in Gaussian.)

  • IRC Calculations
    Cristina Nevado

    Thank you very much for your kind reply.

    In my GRRM17 input file, I specify the option IRC. I see in my file.log "STEEPEST-DESCENT PATH FOLLOWING STARTING FROM NON-STATIONARY POINT" but at the end of the file also I see: "IRC following along both forward and backward directions were finished"

    I converted the file.log  in file.mol to see both movements in gaussview and, at this point, I can see only the forward movement. But as it is indicated in the file.log (IRC following along both forward and backward directions were finished), should doI see also the backward movement?

    Best regards

     

  • Re: IRC Calculations
    Kenichiro Saita

    No. The string "IRC following along both forward and backward directions were finished" just means the IRC calculation was normally terminated (termination message). Your GRRM17 output file (file.log) says that your input structure (pre-optimized TS structure obtained by Gaussian) did not converge in a saddle point in GRRM17. From a non-stationary point, only a mass-weighted steepest-descent path can be computed (the "backward" path cannot be theoretically defined).

    Try SADDLE calculation with "Saddle+IRC" and “MO Guess = filename.chk” options. You will get an actual TS structure and the "forward" and "backward" IRC paths.

    For example:


    # SADDLE/B3LYP/6-31G**

    0 1
    C  -0.079213112255    0.000038936045  -0.592031587948
    O   0.018800714554  -0.000018308130   0.717081381477
    H   0.322739833191   0.909186505065  -0.947006485140
    H   0.322739833191  -0.909359858691  -0.946723086445
    Options
    Saddle+IRC
    GauProc = 4
    GauMem = 100
    MO Guess = filename.chk

  • Re: IRC Calculations
    afiradmin

    This issue seems to be solved by the above comment. (However, you can still add comments to this thread.)