Table 1.

Total number of function/gradient calls of geometry optimisation for organic molecules in gas phase using conventional (ID) and FF-based preconditioned (FF) LBFGS method on three different quantum chemistry surfaces. Convergence threshold was $\parallel \nabla E{\parallel }_{\infty }={10}^{-4}$ eV Å⁻¹ for PM6 and $\parallel \nabla E{\parallel }_{\infty }={10}^{-3}$ eV Å⁻¹ for DFT and MP2 potentials, respectively.

System (# of atoms)	PM6		DFT(BLYP)			MP2/6–31 G**
	ID	FF/GAFF	ID	FF/GAFF	ID	FF/GAFF	FF/LFF	Lindh
5-nitrobenzisoxazole (16)	89	25	119	63	71	27	31	32
menthone (29)	207	29	197	48	107	22	31	38
alanine tripeptide (32)	395	77	536	124	210	59	47	67
thc (53)	720	84	239	69
heme (75)	500	175	358	95
taxol (113)	1662	419
16-mer polyalanine (172)	3549	348