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Calculation Of Intrinsic Reaction Coordinates

Intrinsic reaction coordinates (IRC) can be calculated using a transition state ZMAT with the %irc keyword in the additional keyword section. The starting geometry can be given in internal ZMAT coordinates or cartesian coordinates. For non-default CFOUR input options such as cartesian coordinates or geometry parameters in Angstrom the appropriate keywords have to be supplied in the *CFOUR section. Additionally, all standard CFOUR keywords can be used in the *CFOUR section to specify level of theory, basis sets, etc. The irc driver is not compatible with any other % keyword and these should be avoided. The reaction path will always be followed in mass-weighted cartesian coordinates. The code will copy the starting ZMAT to the zmat0 file. The ZMAT file is updated along the path. The algorithm will use the zmat0 file as the starting ZMAT if available to avoid using a remnant ZMAT from a previous calculation. Therefore the zmat0 file either has to be changed directly or has to be deleted if the starting ZMAT is to be changed. A calculation can also be restarted (for example if the maximum number of points was exceeded) by using a ZMAT along the path from the ZMATs folder and deleting zmat0. Any structure along the IRC path can be used as the starting point. When restarting a calculation or when using a structure along the IRC as the starting structure no starting step is necessary. The gradient and hessian at the transition state or the starting point can be supplied to the program using the GRD, FCM and NORMCO files if these derivatives have already been calculated in a standard CFOUR calculation (for example in a VIB=ANALYTIC calculation to ensure that a first-order saddle point has been found). These files will be saved to grd0, fcm0 and normco0 at the beginning and will be used in case the calculation is restarted. If no files are provided they will automatically be calculated by the algorithm. The zmat0, grd0, normco0 and fcm0 files have to be deleted if new input files are to be supplied. A standard IRC calculation outputs five different file types:

  • the ZMAT files used along the reaction path, located in the zmat_files folder
  • the irc file provides point number, energy, energy difference to the starting geometry, the IRC length and the RMS cartesian gradient
  • the molden_irc file in xyz-format that can be used to visualize the atomic movement along the reaction path with MOLDEN or JMOL
  • the MOLDEN files along the reaction path, located in the molden_files
  • the tunnex_irc file for the calculation of tunneling half-lives using the Wentzel-Kramers-Brillouin (WKB) method with Tunnex

Additionally projected frequencies can be calculated along the reaction path with the appropriate keyword. Two additional outputs are created:

  • the projfreq file provides zero-point vibrational energies and projected frequencies at every step for which projected frequencies were requested
  • the tunnex_zpve file provides the zero-point vibrational energy for Tunnex calculations

The following keywords can be used to control the path-following algorithm and should be specified below the %irc keyword. These keywords can be separated by any standard CFOUR delimiter or by a line break.


  • EULER(=0)
  • IMK(=1)
  • LQA(=2)
  • HPC(=3) default


The stepsize is STEPSIZE • 10-2 amu½ bohr. Default: 10


Direction of starting step along the transition vector at the transition state. Not relevant when START=2.

  • BOTH(=0) default
  • POSITIVE(=1)
  • NEGATIVE(=2)


Convergence criterium for the cartesian RMS gradient is 10-CONV Hartree/bohr. Default: 4


Maximum number of IRC points to be calculated per IRC. Default: 100


  • STEPSIZE(=0): Starting step size is STARTING_STEP • 10-2 amu½ bohr, default
  • ENERGY(=1): Starting step size is chosen so that energy is lowered by STARTING_STEP • 10-5 Hartree
  • NONE(=2): Should only be chosen if calculation starts along the IRC and not at the transition state


  • START=0: STARTING_STEP • 10-2 amu½ bohr. Default: 10
  • START=1: STARTING_STEP • 10-5 Hartree. Default: 20


Step along the bisector of the gradient at the start of the IMK step and after an Euler step in BISECTOR_STEP • 10-3 amu½ bohr. Only relevant for METHOD=1. Default: 25


Specifies what hessian update scheme is used. Only relevant if METHOD=2 or METHOD=3.

  • NONE(=0)
  • BOFILL(=1) default
  • PSB(=2)
  • MS(=3)


Specifies how often the analytical hessian is recalculated. Only relevant if METHOD=2 or METHOD=3. Default: 5


Maximum RMSD of the updated hessian before the hessian is recalculated in HESSIAN_RMSD • 10-3 Hartree / Bohr2. Default: 10


Specifies whether projected frequencies are supposed to be calculated along the reaction path. The reaction path tangent and the translational and rotational degrees of freedom are projected out from the mass-weighted Hessian.

  • NO(=0) default
  • YES(=1)


Specifies the interval of IRC points at which projected frequencies are calculated. The Hessian should be calculated analytically at these points and can be calculated using Hessian updating at the points in between. Default: 5

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Page last modified on March 29, 2021, at 10:25 AM
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CFOUR is partially supported by the U.S. National Science Foundation.