All of the semiempirical methods (MNDO,
AM1, PM3, PM6,
RM1, PM7) underestimate the barrier to
rotation of a peptide bond. A Molecular Mechanics correction has been
added which increases the barrier in N-methyl acetamide to 14 kcal/mol.
If you do not want this correction, specify NOMM (NO Molecular
Mechanics).