Why does Chem3D give different value for minimized energy?

[Chem_Mod]

For gauche, you indicated that the minimized energy of the structure of butane is 0.9 kcal/mol. However, using chem 3D, the calculated minimized energy for the gauche confirmation of butane was around 3 kcal/mol. What accounts for this difference?

[Chem_Mod]

If you remember, the 0.9 value represents the energy difference between gauche and anti config not the total free energy. Think about what the MM2 energy minimization is doing and it should become more clear why the values are not the same.