The Eigenvector Following routine [7] is the default geometry optimization method. For systems with more than 2000 variables, the L-BFGS optimizer is the default.  To prevent the L-BFGS optimizer being used, add keyword EF.  That is, EF is provided to allow explicit definition of the optimizer. Alternative geometry optimizers are the BFGS [19,20,21,22] and the low-memory method, L-BFGS . See also DDMAX, DDMIN, DMAX, HESS, IUPD, LET, MODE=, NONR, OMIN, PRNT=, RECALC, RMAX, RMIN, RSCAL.

The current EF, while very reliable, is sometimes slow. To over-ride some safety checks, specify LET. This will sometimes make the job run faster.