IRC Keyword
Last Update: 12/31/2000

This method keyword requests that a reaction path be followed [103-104]. 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. By default the forward direction is defined as the direction the transition vector is pointing when the largest component of the phase is positive; it can be defined explicitly using the Phase option.

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. The path can be computed in mass-weighted internals, Cartesians or internals coordinates. By default, an IRC calculation steps 6 points in mass-weighted internals in the forward direction and 6 points in the reverse direction, in steps of 0.1 amu^1/2 bohr along the path.

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 IRC=RCFC in the route section. The other possibilities are providing the force constants in the input stream (IRC=FCCards) and computing them at the beginning of the IRC calculation (IRC=CalcFC). Note that one of RCFC, CalcFC and FCCards must be specified.

IRC calculations accept Z-matrices and Cartesian coordinates as molecule specifications and uses these coordinates in following the reaction path.

IRC studies are not currently archived.

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
Follow the reaction path downhill from a starting point that is not a transition state. The gradient for the starting structure must be larger than the optimization convergence threshold. Downhill cannot be used with Forward or Reverse.

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

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

StepSize=N
Step size along the reaction path, in units of 0.01 amu^-1/2-Bohr. The default is 10.

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

FreqCalculates the projected vibrational frequencies for motion perpendicular to the path, for each optimized point on the path [303]. This option is valid only for reaction paths in mass-weighted internal coordinates.

MassWeighted
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.

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

Cartesian
Follow the path in Cartesian coordinates without mass weighting.

ReadIsotopes
Specify alternate isotopes (the defaults are the most abundant isotopes). This information appears in a separate input section having the format:

isotope mass for atom 1
isotope mass for atom 2
...
isotope mass for atom n

where the 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 mass (e.g., 18 specifies O¹⁸, and Gaussian 98 uses the value 17.99916).

RCFC
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.

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

CalcAll
Specifies that the force constants be computed at every point.

FCCards
Reads the Cartesian forces and force constants from the input stream after the molecule specifications. This is used to read force constants recovered from the Quantum Chemistry Archive using its internal FCList command. The format for this input is:

Energy (format D24.16)
Cartesian forces (lines of format 6F12.8)
Force constants (lines of format 6F12.8)

The force constants are in lower triangular form: ((F(J,I),J=1,I),I=1,NAt3), where NAt3 is the number of Cartesian coordinates. If both FCCards and ReadIsotopes are specified, the masses of the atoms are input before the energy, Cartesian gradients and the Cartesian force constants.

OPTIMIZATION ALGORITHM-RELATED OPTION

VeryTight
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.

Restart
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.

HF, all DFT methods, CIS, MP2, MP3, MP4(SDQ), CID, CISD, CCD, QCISD, CASSCF, and all semi-empirical methods.

Opt, Scan, IRCMax

The output for each step of an IRC calculation is very similar to that from a geometry optimization. Each step is introducted by this banner line (where “IRC” has replaced “Grad”):

IRC-IRC-IRC-IRC-IRC-IRC-IRC-IRC-IRC-IRC-IRC-IRC-IRC-IRC-IRC-IRC-IRC

As the optimization at each point completes, the optimized structure is displayed:

 Optimization completed. 
    -- Optimized point #   1 Found. 
                     ----------------------------
                     !   Optimized Parameters   !
                     ! (Angstroms and Degrees)  !
 --------------------                            --------------------
 !      Name          Value   Derivative information (Atomic Units) !
 !                                                                  !
 --------------------------------------------------------------------
 !       CH1         1.3448   -DE/DX =    0.0143                    !
 !       HH          0.8632   -DE/DX =   -0.0047                    !
 !       CH2         1.0827   -DE/DX =    0.0008                    !
 !       HCH       106.207    -DE/DX =   -0.0082                    !
 --------------------------------------------------------------------
   RADIUS OF CURVATURE =    0.39205
   NET REACTION COORDINATE UP TO THIS POINT =    0.09946

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

 --------------------------------------------------------------------
                 SUMMARY OF REACTION PATH FOLLOWING: 
                 (Int. Coord:  Angstroms, and Degrees)
 --------------------------------------------------------------------
                    ENERGY   RX.COORD     CH1       HH        CH2
       1          -40.16837  -0.49759   1.54387   0.73360   1.08145
       2          -40.16542  -0.39764   1.49968   0.74371   1.08164
       3          -40.16235  -0.29820   1.45133   0.76567   1.08193
       4          -40.15914  -0.19914   1.39854   0.80711   1.08232
       5          -40.15640  -0.09946   1.34481   0.86318   1.08274
       6          -40.15552   0.00000   1.30200   0.91500   1.08300
       7          -40.15649   0.09990   1.26036   0.96924   1.08330
       8          -40.15999   0.19985   1.21116   1.03788   1.08349
       9          -40.16486   0.29975   1.16418   1.10833   1.08353
      10          -40.16957   0.39938   1.12245   1.18068   1.08328
      11          -40.17324   0.49831   1.09260   1.25158   1.08276
 --------------------------------------------------------------------
  TOTAL NUMBER OF GRADIENT CALCULATIONS:     28
  TOTAL NUMBER OF POINTS:     10
  AVERAGE NUMBER OF GRADIENT CALCULATIONS:     2.80000

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

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