The small improvement in accuracy of PM6 does not seem to justify its use over AMOEBA, which is about 180 times faster. From these tests on the GPGG tetrapeptide, AMOEBA is thus emerging as a promising surrogate for GA optimization of peptides in terms of cost and accuracy and it will therefore be our choice in the following sections along with the fast but presumably less accurate GAFF (HF/6-31G(d) RESP) force field for comparison.