HESSEI — Compute a few lowest Hessian eigenvalues.

HESSEI=`n`

HESSEI is useful for computing a few of the lowest Hessian eigenvalues in order to
characterize a stationary point. (See Chapter 4, *Computational Procedures* for a
discussion of stationary point
characterization.) HESSEI is faster and more efficient than
FORCE because only a few eigenvalues need to be computed. This is
similar to LFORCE but does not compute
and report IR frequencies. HESSEI will compute all negative and zero eigenvalues plus the
lowest n (1 by default) positive eigenvalues.

Some care must be taken when characterizing stationary points on geometries with frozen variables. FORCE causes all geometric variables to be unfrozen and so may produce unexpected results. For example, an optimization with frozen variables followed by a FORCE calculation may fail because the constrained extrema may not correspond to a genuine extrema once the variables are unfrozen. The FORCE calculation can be made to proceed anyways by using LET, however, the resulting frequencies may not be meaningful. Alternatively, specifying LFORCE (instead of FORCE) will compute a few lowest Hessian eigenvalues and corresponding IR frequencies on the unconstrained geometry, without regard to the gradient norm. A third alternative is to use HESSEI, which computes the lowest Hessian eigenvalues (similar to LFORCE) but does so on the constrained geometry (unlike FORCE and LFORCE).

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