Hypertext Dalton 2.0 manual - Theoretical Chemistry, KTH

CHAPTER 7. POTENTIAL ENERGY SURFACES 55
wave function and then be read in. The minimal input for a transition state optimization
using *OPTIMIZE is:
**DALTON INPUT
.OPTIMIZE
*OPTIMIZE
.SADDLE
**WAVE FUNCTIONS
.HF
**END OF DALTON INPUT
This will calculate the Hessian at the initial geometry, then update it using Bofill’s update
[35] (the BFGS update is not suitable since it tends towards a positive definite Hessian).
As for minimizations, redundant internal coordinates are used by default for first-order
methods.
It is highly recommended that a Hessian calculation/vibrational analysis be performed
once a stationary point has been found, to verify that it’s actually a first-order
transition state. There should be one and only one negative eigenvalue/imaginary frequency.
If no analytical second derivatives (Hessians) are available, it is still possible to
attempt a saddle point optimization by starting from a model Hessian indicated by the
keyword .INIMOD:
**DALTON INPUT
.OPTIMIZE
*OPTIMIZE
.INIMOD
.SADDLE
**WAVE FUNCTIONS
.HF
**END OF DALTON INPUT
Provided the starting geometry is reasonably near the transition state, such optimization
will usually converge correctly. If not, it is usually a good idea to start from different
geometries and also to try to follow different Hessian modes, as described in section 7.1.2
(through the .MODE keyword).
7.1.4 Transition states following a gradient extremal
Reference literature:
P.Jørgensen, H.J.Aa.Jensen, and T.Helgaker. Theor.Chem.Acta, 73, 55,
(1988).