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.)