Alternative Algorithms

The default algorithm for searching for an energy minimum (which you will get if you just use the keyword OPTIMISE) is a version of the BFGS method. BFGS stands for Broyden-Fletcher-Goldfarb-Shanno and is one of the standard quasi-Newton search methods. (It would actually be more accurate to say that Cadpac uses the Broyden rank 2 update, of which the BFGS update is a special case). The version in Cadpac uses more or less the standard procedure in conjunction with some techniques for adjusting the step lengths ('trust radii'). The BFGS method is generally robust (i.e. it does not make many mistakes) and reasonably efficient.

There are two other methods programmed in Cadpac. The optimisation keyword may be modified as follows,

OPTIMISE MINIMUM method

where method is one of the following character strings ,

BFGS

(the default)

AUGHES

This causes use of an augmented hessian technique, and

GDIIS

which causes use of Pulay's DIIS method as applied to geometry searches.

Generally speaking we have found little difference in the performance of all these methods, except that in cases where the initial guess at the geometry is rather poor, then the augmented hessian technique seems to converge more rapidly in the initial stages - its overall performance is however no better than BFGS.