Gaussian 03 Online Manual
Last update: 2 October 2006
IRC
This method keyword requests that a reaction path be followed [151,152]. 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 amu1/2 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, CalcAll and FCCards must be specified.
IRC calculations accept Z-matrices or Cartesian coordinates as molecule specifications and uses these coordinates in following the reaction path.
You should specify alternative isotopes for IRC jobs using the standard method.
IRC studies are not currently archived.
PATH SELECTION 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.
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 amu1/2-Bohr. The default
is 10.
MaxCyc=N
Sets the maximum number of steps in each
geometry optimization. The default is 20.
COORDINATE SYSTEM SELECTION OPTIONS
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.
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 option can be 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 OPTION
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, CCSD, QCISD, CASSCF, and all semi-empirical methods.
The output for each step of an IRC calculation is very similar to that from a geometry optimization. Each step is introduced 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.