46.2.6 Hessian approximations (HESSIAN)
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
- Use Lindh's Model Hessian in optimization (default).
- Use Schlegel's Model Hessian.
- Add vdW terms to Lindh's Model Hessian.
- same as MODEL=SCHLEGEL.
- same as MODEL=VDW.
- Don't use Model Hessian approximation to the hessian.
- 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
- 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.
- Method used for hessian update. See section 46.2.9 for possibilities and details.
- 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