IRC keyword

In Gaussian, a reaction path calculation is requested with the IRC keyword in the route section. Before you can run one, however, certain requirements must be met. An IRC calculation begins at a transition structure and steps along the reaction path a fixed number of times (the default is 6) in each direction, toward the two minima that it connects. However, in most cases, it will not step all the way to the minimum on either side of the path. 

Perform the IRC calculation (requested with the IRC keyword). This job will help you to verify that you have the correct transition state for the reaction when you examine the structures that are downhill from the saddle point. In some cases, however, you will need to increase the number of steps taken in the IRC in order to get closer to the minimum the AAoxPoinb option specifies the number of steps to take in each direction as its argument. You can also continue an IRC calculation by using the lRC=(ReStorhMaxPointe=n) keyword, setting n to some appropriate value (provided, of course, that you have saved the checkpoint file). 

These options to the IRC keyword increase the maximum number of points on each side of the path to 15 and the step size between points to 0.3 amu bohr (30 units of 0.1 amu bohr), where the defaults are 6 steps and 0.1 amu bohr, respectively. The SCF=QC keyword requests the quadratic convergence SCF procedure, a somewhat slower but significantly more reliable SCF procedure. 

These options to the IRC keyword increase the maximum number of points on each side of the path to 15 and the step size between points to 0.3 amu bohr (30 units of... 

We will also use the results of the frequency job in the IRC calculation we ll do next. This job will enable us to verify that this transition structure connects the two minima that we think it does, and we use the keyword IRC to request it. By default, the calculation takes 6 steps in each direction, where each step corresponds to a g. jinetry optimization. However, the calculation will stop searching in a given direction once its convergence criteria are met, and an IRC calculation does not necessarily step all the way down to the minimum. 

Include these keywords in the route section of the IRC calculation ... 

The keyword IRC causes Gaussian to perform an IRC calculation. The input to such a job is the geometry of the transition state. 

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