40.2.6 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 40.2.7).

HESSIAN,options

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. 40.2.7. 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 40.2.7 for more details about numerical Hessians.

UPDATE=type

Method used for hessian update. See section 40.2.9 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 40.2.8.

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.