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.
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 HPC algorithm).
In Gaussian 16, most calculations use the HPC algorithm [Hratchian04a, Hratchian05a, Hratchian05b] by default (introduced in Gaussian 09). It is much more efficient than the one used in earlier program versions. ONIOM(MO:MM) calculations use the Euler predictorcorrector integration algorithm. This same integrator is also used by default in calculations using methods with gradients but without analytic second derivatives; such calculations should include the GradientOnly option. Available algorithms are discussed in Availability.
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 (described in Options).
Path Specification Options
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.
Forward
Follow the path only in the forward direction.
Reverse
Follow the path only in the reverse direction.
Downhill
Proceed downhill from the input geometry.
MaxPoints=N
Number of points along the reaction path to examine (in each direction if both are being considered). The default is 10.
StepSize=N
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 amu^{1/2}Bohr. The default is 10.
ReadIsotopes
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 (the Iso parameter), as in this example:
#T Method/631G(d) JobType Temperature=300.0 …
…
0 1
C(Iso=13)
…
ReadIsotopes input has the following format:
temp pressure [scale] 
Values must be real numbers. 
isotope mass for atom 1


isotope mass for atom 2


…


isotope 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 ^{18}O, and Gaussian uses the value 17.99916).
The default algorithms are available for HF, all DFT methods, CIS, TD, MP2, MP3, MP4(SDQ), CID, CISD, CCD, CCSD, QCISD, BD, CASSCF, and all semiempirical methods.
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 transition state
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.