1: LBFGS

This line tells cluster what to do after it has solved the self consistent equations for the electron density. The default is LBFGS. This makes cluster calculate the forces acting on the nuclei in the molecule and adjusts the positions accordingly. This new geometry is written to the SYMBOL file as a new calculation. The option CONJUGATE-GRADIENT does the same thing in a different manner. The other option for this line is SCF-ONLY. This causes cluster not to calculate a new geometry. This is used when you generate a SYMBOL file with more one calculation, or don't want to do a geometry optimisation.