Table 2.

Comparison of total number of function/gradient calls of geometry optimisation of different optimisation algorithms for minimisation of small organic molecules on PM6 surface: unpreconditioned LBFGS (ID), FF-based preconditioned LBFGS (FF/GAFF), FF-Hessian based preconditioned LBFGS (Hessian/GAFF), PM6-Hessian based preconditioned LBFGS (PM6/GAFF). Convergence threshold was $\parallel \nabla E{\parallel }_{\infty }={10}^{-4}$ eV Å⁻¹ for all cases.

System (# of atoms)	LBFGS
	ID	FF/GAFF	Hessian/GAFF	Hessian/PM6
5-nitrobenzisoxazole (16)	89	25	32	24
menthone (29)	207	29	36	21
alanine tripeptide (32)	395	77	109	110
thc (53)	720	84	95	120