Intrinsic Reaction Coordinate (IRC)
In reaction mechanism studies, once a Transition State (TS) structure has been optimized
(a first-order saddle point characterized by exactly one imaginary frequency), it is essential
to confirm that this TS genuinely connects the expected reactants and products.
The Intrinsic Reaction Coordinate (IRC) calculation traces the Minimum Energy Path (MEP)
in mass-weighted coordinates downhill from the transition state in both forward and reverse directions.
Prerequisites for an IRC:
1. A fully converged Transition State structure (`! OptTS`).
2. A frequency calculation on that TS yielding one imaginary frequency corresponding to the reaction vector.
3. The resulting Hessian file (*.hess), which ORCA reads to define the initial displacement vector along the imaginary mode.
In ORCA, the IRC calculation is controlled with the ! IRC keyword and the %irc block.
Once finished, ORCA produces trajectory files (*_IRC_F.xyz, *_IRC_B.xyz, and *.irc.xyz)
that can be animated in wxMacMolPlt, Chemcraft, or Avogadro to observe the complete reaction step by step.
Key IRC keywords and block parameters:
! IRC Instructs ORCA to run an Intrinsic Reaction Coordinate path calculation.
Direction Both Integrates the reaction path downhill in both forward and backward directions.
InitHess Read Reads the initial Hessian matrix from the previous TS frequency calculation.
Hess_Filename Specifies the .hess file containing the imaginary frequency transition vector.
MaxSteps Sets the maximum number of integration steps taken in each direction along the path.
wxmacmolplt file.irc.xyz Visualizes the animated reaction coordinate trajectory.
[emma@pople]$ vi irc_calc.inp
# ORCA Intrinsic Reaction Coordinate (IRC) Calculation
! RKS PBE TZVP
! TightSCF PAL8
! IRC
%irc
Direction Both # Follow both directions
MaxSteps 30 # Points along the path
InitHess Read # Read TS Hessian
Hess_Filename "ts_freq.hess"
end
%method
Grid 2 #110 grid points for the SCF
FinalGrid 4 #302 grid points
end
#Type Charge Multiplicity (TS Coordinates)
* xyz 0 1
O -0.0010 -0.0190 0.3160
H 0.9810 0.0140 0.4840
C -0.4360 -1.2530 0.2830
O -1.5790 -1.5460 0.0170
C -0.1520 1.3500 -0.2100
H -1.1800 1.6200 -0.1500
*
[emma@pople]$ /path/to/orca irc_calc.inp > irc_calc.out
[emma@pople]$ ls *IRC*.xyz
irc_calc_IRC_F.xyz irc_calc_IRC_B.xyz irc_calc.irc.xyz
[emma@pople]$ wxmacmolplt irc_calc.irc.xyz
Alternative for torque / PBS users
[emma@pople]$ qsub orca-4.2.0.pbs
FamaLab tutorial