Gaussian 03 Online Manual
Last update: 2 October 2006
IRCMax
Performs an IRCMax calculation using the methods of Petersson and coworkers [168,169,170,171,172,173,174,175,176]. Taking a transition structure as its input, this calculation type finds the maximum energy along a specified reaction path.
You should specify alternative isotopes for IRCMax jobs using the standard method.
REQUIRED INPUT
IRCMax requires two model chemistries as its options, separated by a colon: IRCMax(model2:model1). Here is an example route section:
# IRCMax(B3LYP/6-31G(d,p):HF/6-31G(d,p))
This 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.
The Zero option will produce the data required for zero curvature variational transition state theory (ZC-VTST) [169,170,173,174,175,176]. Consider the following route:
# IRCMax(MP2/6-31G(d):HF/3-21G*,Zero,Stepsize=10)
This job will start from the HF/3-21G* TS and search along the HF/3-21G* IRC with a stepsize of 0.1 amu1/2 bohr until the maximum of the MP2/6-31G(d) energy (including the HF/3-21G* ZPE scaled by 0.91671) is bracketed. The position along the HF/3-21G* IRC for this MP2/6-31G(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/3-21G*), the imaginary frequency for tunneling (fit to MP2/6-31G(d) + ZPE), and the total MP2/6-31G(d) + ZPE energy of the TS.
ZC-VTST OPTIONS
Zero
Include the zero-point energy in the IRCMax computation.
PATH SELECTION OPTIONS
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.
Freq
Calculate the
projected vibrational frequencies for motion perpendicular to the path, for each
optimized point on the path [496]. This option
is valid only for reaction paths in mass-weighted internal coordinates.
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.
CONVERGENCE-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.
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.
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.
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.
