LBFGS

Optimize the geometry using the L-BFGS-B function minimizer. This is based on the BFGS optimizer, but it does not store the inverse Hessian, instead it is calculated as needed. Because of this, the L-BFGS-B method uses very little storage, and is therefore suitable for optimizing very large systems.

The L-BFGS optimizer is the default if 100 or more variables are to be optimized.  For such systems, L-BFGS is more efficient than EF.  If L-BFGS is not wanted, add keyword EF