46.2.6 Hessian approximations (

By default, the MOLPRO geometry optimization utilizes a force field approximation to the hessian
(“Model Hessian”, see R. Lindh, A. Bernhardsson, G. Karlström and P. Malmqvist
*Chem. Phys. Lett.* **241**, 423 (1995)), which speeds up convergence significantly.
The Model Hessian is parameterized for the elements up to the third row.
Alternatively, the model Hessian of
Schlegel can be used, or the Hessian can be computed numerically (see also section 46.2.7).

where *options* can be

`MODEL`- Use Lindh's Model Hessian in optimization (default).
`MODEL=SCHLEGEL`- Use Schlegel's Model Hessian.
`MODEL=VDW`- Add vdW terms to Lindh's Model Hessian.
`SCHLEGEL`- same as
`MODEL=SCHLEGEL`. `VDW`- same as
`MODEL=VDW`. `NOMODEL`- Don't use Model Hessian approximation to the hessian.
`NUMERICAL`=*hstep*- Recompute Hessian after
*hstep*iterations. This disables the use of a model hessian. If*hstep*=0, the Hessian is only computed in the first iteration. Default parameters are used for computing the numerical Hessian, unless modified using options as described for the`NUMHESS`directive, see Sect. 46.2.7. Any option valid for the`NUMHESS`directive may also follow the`NUMERICAL`option on the`HESSIAN`directive. `READ|RECORD|HESSREC`=*record*- Read Hessian from given
*record*. If*record*is not given or zero, the last computed hessian will be read. See section 46.2.7 for more details about numerical Hessians. `UPDATE`=*type*- Method used for hessian update. See section 46.2.9 for possibilities and details.
`MAXUPD`=*maxupd*- Max number of hessian updates. The count is reset to zero each time a hessian is computed.

If the Model Hessian is disabled (`NOMODEL`) and no Hessian is read or computed,
the initial hessian is assumed to be diagonal, with values 1 hartree*bohr**(-2)
for all lengths, 1 hartree*radian**(-2) for all angles.
Additional matrix elements of the hessian can be defined using the HESSELEM directive, see section 46.2.8.

In transition state searches the Hessian is evaluated numerically
in the first iteration by default. Alternatively, if `READ` is specified, a previously computed hessian
is used.

molpro@molpro.net 2019-03-25