LFORCE — Compute the IR spectrum for a few lowest frequencies.

LFORCE=`n`

LFORCE provides a fast and efficient alternative to
FORCE when only a few of the
lowest frequencies are required. Like
FORCE, it begins by computing the Hessian (that is the matrix in millidynes per Angstrom of
second derivatives of the energy with respect to displacement of all pairs of atoms in x, y,
and z). Rather than diagonalizing the force matrix (the Hessian matrix appropriately weighted
with isotopic masses) directly, LFORCE iteratively solves for a small subset of force
constants, which are then used to compute vibrational frequencies. By avoiding the expensive
diagonalization, LFORCE is generally much faster than the corresponding FORCE calculation.
LFORCE will compute all negative and zero frequencies plus the lowest n (1 by default)
positive IR frequencies. This is useful if one only needs to characterize a stationary point.
Before a LFORCE calculation is performed, the gradient norm (gnorm) is computed for the input
geometry, and if this value is larger than 3.00, the computation is halted. See
Chapter 4, *Computational Procedures* for a
discussion of stationary point
characterization.

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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