This calculation type keyword requests that a reaction path be followed by integrating the intrinsic reaction coordinate [Fukui81, Hratchian05a]. The initial geometry (given in the molecule specification section) is that of the transition state, and the path can be followed in one or both directions from that point. The forward direction is defined as the direction the transition vector is pointing when the largest component of the transition vector (“phase”) is positive; it can be defined explicitly using the Phase option. By default, both reaction path directions are followed. Gaussian 09 uses a new algorithm [Hratchian04a, Hratchian05a, Hratchian05b] for computing points on the reaction path which is much more efficient than the one used in earlier program versions. See the discussion of the HPC option below.

IRC calculations require initial force constants to proceed. You must provide these to the calculation in some way. The usual method is to save the checkpoint file from the preceding frequency calculation (used to verify that the optimized geometry to be used in the IRC calculation is in fact a transition state), and then specify the RCFC option in the route section. Another possibility is to compute them at the beginning of the IRC calculation (CalcFC). Note that one of RCFC and CalcFC must be specified (CalcAll is also available but is not typically necessary with the new IRC algorithm).

In Gaussian 09, the default IRC algorithms have changed. Most calculations use the HPC algorithm by default. ONIOM(MO:MM) calculations use the Euler predictor-corrector integration algorithm. Calculations using methods with gradients but without analytic second derivatives should include the GradientOnly option and will then use the damped velocity verlet integrator (DVV), but may specify Euler integration instead if desired (Euler). Available algorithms are discussed in detail below.

The default is to report only the energies and reaction coordinate at each point on the path; if geometrical parameters along the path are desired, these should be defined as redundant internal coordinates via Geom=ModRedundant or as input to the IRC code via IRC(Report=Read).

You can specify alternative isotopes for IRC jobs using the ReadIsotopes option (see below).


Phase=(N1, N2 [,N3 [,N4]])
Defines the phase for the transition vector such that forward motion along the transition vector corresponds to an increase in the specified internal coordinate, designated by up to four atom numbers. If two atom numbers are given, the coordinate is a bond stretch between the two atoms; three atom numbers specify an angle bend; and four atoms define a dihedral angle.

Follow the path only in the forward direction.

Follow the path only in the reverse direction.

Proceed downhill from the input geometry.

Number of points along the reaction path to examine (in each direction if both are being considered). The default is 10.

Step size along the reaction path, in units of 0.01 Bohr. If N<0, then the step size is taken in units of 0.01 amu1/2-Bohr. The default is 10.

This option allows you to specify alternatives to the default temperature, pressure, frequency scale factor and/or isotopes—298.15 K, 1 atmosphere, no scaling, and the most abundant isotopes (respectively). It is useful when you want to rerun an analysis using different parameters from the data in a checkpoint file.

Be aware, however, that all of these can be specified in the route section (Temperature, Pressure and Scale keywords) and molecule specification (Iso= parameter), as in this example:

#T Method/6-31G(d) JobType Temperature=300.0 …


0 1
ReadIsotopes input has the following format:
temp pressure [scale]   Values must be real numbers.
isotope mass for atom 1
isotope mass for atom 2isotope mass for atom n

where temp, pressure, and scale are the desired temperature, pressure, and an optional scale factor for frequency data when used for thermochemical analysis (the default is unscaled). The remaining lines hold the isotope masses for the various atoms in the molecule, arranged in the same order as they appeared in the molecule specification section. If integers are used to specify the atomic masses, the program will automatically use the corresponding actual exact isotopic mass (e.g., 18 specifies 18O, and Gaussian uses the value 17.99916).


Use the Hessian-based Predictor-Corrector integrator [Hratchian04a, Hratchian05a, Hratchian05b]: a very accurate algorithm that uses the Hessian-based local quadratic approximation as the predictor component and a modified Bulrisch-Stoer integrator for the corrector portion. This corrector integrator is done using a distance weighted interpolant surface [Collins02] fitted to energies, gradients, and Hessians at the beginning and ending points of the predictor step. This is the default for most calculations. Note that it is not practical for extremely large molecular systems.

Use the first-order Euler integration for the predictor step along with the HPC corrector step. This is the default for IRC=GradientOnly calculations. This is also the default algorithm for IRC calculations using an ONIOM(MO:MM) method. It is a practical choice for such calculations on large molecules.

Use the local quadratic approximation [Page88, Page90].

Use the damped velocity verlet integrator [Hratchian02].

Use only the first-order Euler integration predictor step for the IRC. This option is not recommended for production use.

Compute the Hessian analytically every N predictor steps, or every |N| corrector steps if N<0. Analytic second derivatives can be requested intermittently during IRCs using IRC=(CalcFC,RecalcFC=(Predictor=N,Corrector=M)), which computes second derivatives at the initial point and then at every Nth predictor step and every Mth corrector step. You must still specify RCFC or CalcFC to provide the initial Hessian. Update is a synonym for ReCalc. Requires a method which has analytic second derivatives.

Use an algorithm which does not require second derivatives. Note that you must specify this option explicitly for such methods (they are not automatically detected). Can be combined with EulerPC (the default), Euler or DVV.


Follow the path in mass-weighted Cartesian coordinates. MW is a synonym for MassWeighted. This is the default.

Follow the path in Cartesian coordinates without mass-weighting.


Specifies that the computed force constants in Cartesian coordinates from a frequency calculation are to be read from the checkpoint file. ReadCartesianFC is a synonym for RCFC.

Specifies that the force constants be computed at the first point.


The GS2 option requests the IRC algorithm used in Gaussian 03 within the new IRC implementation. Use the keyword Use=L115 in order to run the code that was the default in Gaussian 03 (recommended for reproducing old results only).

Use the IRC algorithm that was the default in Gaussian 03 and earlier [Gonzalez89, Gonzalez90]. The geometry is optimized at each point along the reaction path such that the segment of the reaction path between any two adjacent points is described by an arc of a circle, and so that the gradients at the end points of the arc are tangent to the path. By default, a GS2 IRC calculation steps 6 points in mass-weighted internal coordinates in the forward direction and 6 points in the reverse direction, in steps of 0.1 amu1/2 Bohr along the path.

Specifies that the force constants be computed at every point.


Restarts an IRC calculation which did not complete, or restarts an IRC calculation which did complete, but for which additional points along the path are desired.

Controls which geometric parameters are reported by an IRC. By default, no geometrical parameters are reported. Report without a parameter includes all of the generated internal coordinates. The possible values for item are:

Read   Read a list of internal coordinates to report. These are specified in the same way as for ModRedundant.
Bonds   Reports bonds from the internal coordinates (if present).
Angles   Reports angles from the internal coordinates (if present).
Dihedrals   Reports dihedrals from the internal coordinates (if present).
Cartesians   Reports all Cartesian coordinates.

Controls testing-and-recomputing for the correction step of HPC and EulerPC IRCs. ReCorrect (without a parameter) and ReCorrect=Yes say to repeat the corrector step whenever the correction is greater than the threshold (which can be decreased with the Tight and VTight options). The parameter can take on the following values:

Never   Do not repeat correction steps (i.e., suppress the threshold test).
Always   Always recompute the corrector at least once regardless of the size of the initial correction.
Test   Test the quality of the corrector step and report the results, but do not take an additional corrector step. The computed IRC path will be the same as with ReCorrect=Never.
The default is Yes for EulerPC and HPC, and Never for other integrators.

Sets the maximum number of steps to N. The default is 20.

This option tightens the cutoffs on forces and step size that are used to determine convergence. For DFT calculations, Int=UltraFine should be specified as well.

Extremely tight optimization convergence criteria. VTight is a synonym for VeryTight. For DFT calculations, Int=UltraFine should be specified as well.


The default algorithms are available for HF, all DFT methods, CIS, MP2, MP3, MP4(SDQ), CID, CISD, CCD, CCSD, QCISD, BD, CASSCF and all semi-empirical methods.


Opt, Scan, IRCMax


When the IRC has completed, the program prints a table summarizing the results:

 Reaction path calculation complete.

 Energies reported relative to the TS energy of         -91.564851
    Summary of reaction path following
                        Energy   Rx Coord
   1                   -0.00880  -0.54062
   2                   -0.00567  -0.43250
   3                   -0.00320  -0.32438
   4                   -0.00142  -0.21626
   5                   -0.00035  -0.10815
   6                    0.00000   0.00000
   7                   -0.00034   0.10815
   8                   -0.00131   0.21627
   9                   -0.00285   0.32439
  10                   -0.00487   0.43252
  11                   -0.00725   0.54065

The initial geometry (transition structure) appears in the middle of the table (in this case, as point 6). It can be identified quickly by looking for reaction coordinate and energy values of 0.00000.


Last update: 3 May 2013