n=0 :

Use a diagonal matrix.

n=1 :

Calculate it numerically (single finite difference of the gradient) (default for TS).

n=2 :

(No longer used)

n=3 :

Calculate it numerically (central finite difference of the gradient).

n=4 :

Restart EF from a previous EF restart file, (jobname.res)

n=5 :

Compute a full Hessian using LTRD in either Cartesian or internal coordinates.

n=6 :

Fast estimate using 2-body model then refine low eigenmodes by Powell Rank 1 Update.

n=7 :

Fast estimate using 2-body model (default for EF).