IRC start direction

As mentioned above, the IRC path is initialized by a first step away from the Transition State. If perfect information is available this should be along the unique Hessian eigenvector with a negative eigenvalue. Therefore, it is preferable to supply (with a restart file) a good approximation of the Hessian at the Transition State. This can be computed in a Frequencies run. In many cases the automatic internally generated (force field based) Hessian will not severely disturb the procedure and may only require a few more initial search steps for the right direction to take, while saving a potentially expensive Frequencies calculation.

If you decide to use a precalculated Hessian, then usually the approximate Hessian resulting from a Transition State run will be good enough. The latter approach is more attractive of course since the TS run will usually be done anyway, as a preliminary to the IRC run, while an additional Frequencies run would be very demanding. At the other hand, Transition State runs often require a preceding Frequencies run. In such case, the Frequencies result file may be used both for the TS run and for the IRC run. The fact that the Frequencies run may have been performed not at the exact TS may affect slightly the adequacy for using it as a start-up for the IRC run, but this is likely not significant.

In some case you may want to specify the initial direction of the IRC path explicitly. This is done as follows:

IRCSTART
 data
 data
 ...
end

IRCstart

A block-type key. The data in the data block are values for all atomic coordinates (Cartesian or Z-matrix, as the case may be) that are not frozen and not (by geovar) explicitly instructed to remain equal. All such coordinate data together define a direction vector in the space of all (free) coordinates, which then serves as the initial segment of the IRC path.

Note that only a direction vector is defined here: the size of the total vector plays no role.

Furthermore, the initial step may be in the positive or negative direction along the so-defined initial path, see the section Forward / Backward IRC paths.