作者Author: fhh2626 时间: 2016-4-27 10:51
Tutorial: Intrinsic Reaction Coordinate (IRC)
Based on Martin Krupicka's forum post.
The Intrinsic Reaction Coordinate (IRC) method is useful to confirm that a located saddlepoint lies on the minimum energy path between 2 assumed minima (instead of it being a saddlepoint for something else). The IRC method is not directly available in ORCA as a simple keyword (yet) but an approximate IRC method can be used as follows.
1. Do a saddlepoint optimization using the eigenvalue-following TS method (OptTS keyword; see usage in Geometry optimizations and in Tutorial: Transition State Optimization ).
2. Confirm that the located saddlepoint corresponds to a 1st order saddlepoint by a frequency calculation (only 1 imaginary frequency should be found).
3. Create a XYZ animation of the imaginary mode using orca_pltvib (using either the filename.hess file of the frequency job or the frequency job outputfile). The imaginary frequency is usually mode number 6 (for non-linear molecules). orca_pltvib will create a 20 frame file (called filename.v006.xyz or something) of the saddlepoint mode that can be visualized using VMD for example.
orca_pltvib filename.hess 6
orca_pltvib filename.out 6
4. In the filename.v006.xyz file, locate 2 frames that are on either "side" of the saddlepoint geometry (note that the saddlepoint geometry appears first and last in the file).
5. Perform a geometry optimization of each frame above. Since you want to make sure that the geometry optimization takes only steps proportional to the negative of the gradient, towards the nearest minimum, it's a good idea to force ORCA to do a Steepest descent optimization. The following input forces use of the unit Hessian and disables Hessian update which effectively turns the Quasi-Newton algorithm into a Steepest descent algorithm:
! Opt
%geom
inhess unit
update noupdate
step qn
end