A Calculation Using Our Forcefield

Torsional contribution cf. structure with no gauche — butane interactions 

Actually, nonbonding interactions are already included in the torsional term (as gauche-butane interactions) we might have used an ethane-type torsional function and accounted for CH3/CH3 interactions entirely with nonbonded terms. However, in comparing calculated relative energies the torsional term will cancel out. 

The stretching and bending terms for structure 2 are the same as for structure 1, except for the contribution of the central C-C bond strictly speaking, the torsional term should be smaller, since the opposing C(CH3) groups have been moved apart. 

---