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Input File

An Intrinsic Reaction Coordinate (IRC) calculation begins from a previously optimized transition state. The purpose of the input file is to instruct Gaussian to follow the reaction pathway in both the forward and reverse directions, thereby connecting the transition state with the corresponding reactants and products.

The complete input file used in this tutorial is shown below.

%nprocshared=1
%mem=2GB
%chk=formicacid-IRC.chk
# irc=(maxpoints=25,calcfc) b3lyp/6-31g geom=connectivity

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0 1
 C                  1.80144200   -0.00030600    0.00000000
 H                  2.89067500   -0.00040700   -0.00000200
 O                  1.21689100   -1.14640400   -0.00000100
 H                 -0.00096800   -1.18741300    0.00000000
 O                  1.21747700    1.14621400    0.00000100
 C                 -1.80143100    0.00030300    0.00000000
 H                 -2.89065900    0.00038900   -0.00000100
 O                 -1.21689400    1.14640700   -0.00000100
 O                 -1.21745900   -1.14621500    0.00000100
 H                  0.00076700    1.18742500    0.00000000

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 6 7 1.0 8 2.0 9 2.0
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Input File Structure

The input file consists of four main sections:

  1. Link 0 Commands
  2. Route Section
  3. Molecular Specification
  4. Connectivity

Most of these sections are identical to previous Gaussian calculations. The primary differences are found in the Route Section, which introduces the IRC-specific keywords.

Link 0 Commands

%nprocshared=1
%mem=2GB
%chk=formicacid-IRC.chk

These commands define the computational resources available to Gaussian.

  • %nprocshared specifies the number of CPU cores.
  • %mem allocates the available memory.
  • %chk creates the checkpoint file used to store wavefunction and geometry information.

These keywords have been discussed in earlier sections.

Route Section

# irc=(maxpoints=25,calcfc) b3lyp/6-31g geom=connectivity

The Route Section contains the instructions that define the IRC calculation.

It specifies

  • the type of calculation,
  • the electronic structure method,
  • the basis set,
  • the molecular geometry format.

Each keyword is explained below.

The irc Keyword

irc

This keyword instructs Gaussian to perform an Intrinsic Reaction Coordinate calculation.

Unlike a Geometry Optimization, which searches for a stationary point, an IRC calculation begins from an already optimized transition state and follows the minimum-energy pathway (MEP) toward the reactants and products.

The maxpoints Option

irc=(maxpoints=25)

The maxpoints option specifies the maximum number of IRC points that Gaussian will generate in each direction along the reaction path.

In this example,

maxpoints=25

means that Gaussian will calculate up to 25 optimization steps while moving toward the reactants and another 25 steps while moving toward the products (subject to convergence).

Increasing this value allows Gaussian to follow longer reaction pathways, whereas smaller values reduce the computational cost.

The calcfc Option

irc=(calcfc)

The calcfc option requests an exact Hessian calculation before the IRC path is initiated.

The Hessian matrix provides information about the curvature of the Potential Energy Surface and improves the stability of the reaction-path following algorithm.

For most IRC calculations, using calcfc is recommended because it produces a more reliable starting direction.

Density Functional Method

b3lyp

The electronic energy at each IRC point is calculated using the B3LYP density functional.

As the molecular geometry changes along the reaction pathway, Gaussian performs a new SCF calculation at every IRC point to determine the corresponding electronic energy.

Basis Set

6-31G

The 6-31G basis set is used to describe the molecular orbitals throughout the reaction pathway.

The same basis set should normally be used for

  • the transition state optimization,
  • the frequency calculation,
  • and the IRC calculation,

to ensure consistency.

Geometry Specification

geom=connectivity

This keyword tells Gaussian to interpret the molecular geometry together with the connectivity information supplied after the Cartesian coordinates.

Using connectivity helps preserve the intended molecular structure while following the reaction path.

Molecular Geometry

The Cartesian coordinates describe the optimized transition-state structure that serves as the starting point for the IRC calculation.

Unlike Geometry Optimization, the IRC calculation does not start from the reactants or products.

Instead, it always begins from the transition state.

Connectivity Section

The connectivity section defines how the atoms are bonded.

Gaussian uses this information to preserve the molecular topology while generating structures along the reaction pathway.

Since this format has been discussed previously, it is not repeated here.

Important Points

Before running an IRC calculation, ensure that:

  • The starting structure is a converged transition state.
  • The transition state has exactly one imaginary frequency.
  • The computational method is the same as that used during the Transition State Search.
  • The checkpoint file corresponds to the transition-state calculation.

Failure to satisfy these conditions may result in an incorrect reaction pathway.

Related Sections

📘 Transition State Search

Frequency Calculation

📘 Frequency Calculation

Summary

The IRC input file is very similar to a standard Gaussian input, with the primary difference being the use of the irc keyword and its associated options. The maxpoints parameter controls the length of the reaction pathway, while calcfc computes an initial Hessian to improve the stability of the path-following algorithm. Together with the chosen electronic structure method and basis set, these keywords enable Gaussian to trace the minimum-energy path connecting the transition state with the reactants and products.