Figure 2.

Torsion energy profiles for rotation around the χ₁ angle in isoleucine. Energy profiles were calculated for three different values of the χ₂ angle, namely (A) −60°, (B) 60°, and (C) 178°. The dihedral angle shown here is defined as N–C^α–C^β–C^γ2. Ab initio energies calculated at the LMP2 level are reported in solid blue lines, whereas Amber ff99SB force field energies are reported in solid black lines. A modified torsion term for the χ₁ angle (see Table I) was introduced to maximize the agreement between the ab initio and the force-field energies. The resulting Amber ff99SB-ILDN energies are reported in solid red lines. The dashed lines show the differences between the QM and molecular mechanics energies (with ff99SB in black dashed lines and ff99SB-ILDN in red dashed lines).