The input needed for doing a level-shifted mode following is very similar to the input for following a gradient extremal , and the keyword that is needed in order to invoke this kind of calculation is .MODFOL . As for gradient extremals, we need to specify which mode we follow. However, a mode following does not use mass-weighted molecular coordinates as default, and isotopic composition of the molecule is therefore not needed. Note, however, that mass-weighted coordinates can be requested through the keyword .MASSES as described in the input section for the *WALK module. A typical input following the third mode will thus look like:
**DALTON INPUT .WALK *WALK .START .MODFOL .INDEX 1 .MODE 3 **WAVE FUNCTIONS .HF *END OF INPUT
The level-shifted mode-following uses an algorithm similar to the one
used in the ordinary geometry optimization of a molecule, but whereas
one in minimizations chooses a step so that the level shift parameter
is less than the lowest eigenvalue of the molecular
Hessian , this
level shift parameter is chosen to be in-between the eigenvalues
and 
if we are following mode number t.
This approach is described was pionered by Cerjan and
Miller [17], and is also described in
Ref. [11].
As for the gradient extremal approach, higher-order transition
states
can be requested throguh the use of the keyword .INDEX
.
Note that it may often be necessary to start the mode-following calculation by stepping out of the stationary point along the mode of interest using the keyword .EIGEN in the .WALK module. We refer to the reference manual for a further description of this option.
The index of the critical point -- that is, the number of negative Hessian eigenvalues -- sought, need to be specified (by the keyword .INDEX ), as the calculation would otherwise continue until a critical point with index zero (corresponding to a minimum), is found.