PATH — Follow the descending reaction path.

PATH=`n.n`

This option follows a weighted steepest descent path in internal coordinates. (A more
detailed description of the PATH
algorithm can be found in Chapter 4, *Computational Procedures*.) Keyword
T.V. or T.V.=n can be used to specify the
initial direction and WEIGHT can be
used to weight the coordinate system. The most common application is to begin at a transition
state geometry and move in the direction of the transition vector. Geometries and properties
will be reported at regular intervals along the path. When PATH is specified alone, the
default spacing between reported points is chosen based on the total number of degrees of
freedom, N, of the system (default spacing = min(0.08*sqrt(N),1.5) Angs). Use
PATH=* n.n* to set this spacing between these reported points to n.n
Angstroms.

IRC differs from PATH in that it runs weighted Cartesian coordinates rather than internal coordinates. Since PATH runs in internal coordinates, it can utilize symmetry constraints, whereas IRC cannot. IRC also relies on computing second derivatives and so is more reliable but also more expensive then PATH. Because IRC (unlike PATH) uses the Hessian, IR frequencies and thermodynamic properties can be computed at each point along the IRC.

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