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Performs an IRCMax calculation using the methods of Petersson and coworkers [Eyring35, Truhlar70, Truhlar71, Garrett80, Malick98, Petersson98, Schwartz98, Petersson98a, Truhlar71, Skodje82]. Taking a transition structure as its input, this calculation type finds the maximum energy along a specified reaction path, using the GS2 algorithm [Gonzalez89, Gonzalez90] (see IRC=GS2 for details).

IRCMax requires two model chemistries as its options, separated by a colon: IRCMax(model2:model1): e.g., IRCMax(B3LYP/6-31G(d,p):HF/6-31G(d,p))

ZC-VTST Options


Include the zero-point energy in the IRCMax computation.

Path Selection Options


Follow the path only in the forward direction.


Follow the path only in the reverse direction.


Read in the vector to follow. The format is Z-matrix (FFF(I), I=1,NVAR), read as (8F10.6).


Number of points along the reaction path to examine (in each direction if both are being considered). The default is 6.


Step size along the reaction path, in units of 0.01 amu1/2-Bohr. The default is 10.


Sets the maximum number of steps in each geometry optimization. The default is 20.

Coordinate System Selection Options


Follow the path in mass-weighted internal (Z-matrix) coordinates (which is equivalent to following the path in mass-weighted Cartesian coordinates). MW is a synonym for MassWeighted. This is the default.


Follow the path in internal (Z-matrix) coordinates without mass-weighting.


Follow the path in Cartesian coordinates without mass-weighting.

Convergence-Related Option


Tightens the convergence criteria used in the optimization at each point along the path. This option is necessary if a very small step size along the path is requested.

Options For Generating Initial Force Constants


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


Specifies that the force constants be computed at every point. The projected vibrational frequencies for motion are calculated for each optimized point on the path [Baboul97]. Note that these projected vibrational frequencies are valid only for reaction paths computed in mass-weighted internal coordinates.

Specifying Alternate Isotopes


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/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 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 18O, and Gaussian uses the value 17.99916).

Analytic gradients are required for the IRC portion of the calculation (model1 above). Any non-compound energy method and basis set may be used for model2.

The following calculation will find the point on the HF/6-31G(d,p) reaction path where the B3LYP/6-31G(d,p) energy is at its maximum:

# IRCMax(B3LYP/6-31G(d,p):HF/6-31G(d,p))

The Zero option will produce the data required for zero curvature variational transition state theory (ZC-VTST) [Eyring35, Truhlar70, Truhlar71, Garrett80, Skodje82, Petersson98, Truhlar71]. Consider the following route:

# IRCMax(MP2/6-311G(d):HF/6-31G(d),Zero,Stepsize=15,CalcAll)

This job will start from the HF/6-31G(d) TS and search along the HF/6-31G(d) IRC with a step size of 0.15 amu1/2 Bohr until the maximum of the MP2/6-311G(d) energy (including the HF/6-31G(d) ZPE) is bracketed. The position along the HF/6-31G(d) IRC for this MP2/6-311G(d) TS will then be optimized. The output includes all quantities required for the calculation of reaction rates using the ZC-VTST version of absolute rate theory: TS moments of inertia, all real vibrational frequencies (HF/6-31G(d)), the imaginary frequency for tunneling (fit to MP2/6-311G(d) + ZPE), and the total MP2/6-311G(d) + ZPE energy of the TS. Note if CalcAll is not used then all these quantities (ZPE, frequencies, etc.) are only computed at the HF/6-31G(d) level and the same quantities are used for all points in the IRCMax path.

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