In Baker's Eigenvector Following routine, options exist for deciding how to construct the initial Hessian matrix. The default is HESS=0. Options available are:

HESS=0

This is the default for geometry optimization (i.e., when EF is used). The initial Hessian is set equal to a diagonal matrix, with the diagonal terms set to 1000 kcal/mol/Å^{2} for bond-lengths, 500 kcal/mol/degrees^{2} for angles, and 200 kcal/mol/degrees^{2} for dihedrals. If Cartesian coordinates are used, all diagonal elements are set to 200 kcal/mol/Å^{2}.

Do not specify HESS=0 unless there is a good reason to do so.

HESS=1

This is the default for transition-state location (i.e., when TS is used). The full Hessian matrix is constructed using single-sided derivatives, see
Hessian Matrix, using the same density matrix throughout the entire construction of the Hessian.

HESS=2

A rarely used option. The Hessian matrix from an earlier run can be used to start the current job. In order for this to work, there must be a one-to-one correspondence of parameters to be optimized. For example, if a geometry optimization using EF and AM1 were to be followed by a similar geometry optimization using EF and PM3, then the Hessian from the earlier calculation could be used to start the PM3 job. (A simpler way of achieving this result is to use RESTART, but note that this will also use the old geometry.)