Jonathan M Goodman Department of Chemistry, Lensfield Road, Cambridge Studies on short alkanes show that the global minima for chains of less than eighteen carbon atoms are extended conformations. Longer chains prefer hairpin-like conformations.
Longer chains are likely to have more complicated global minima structures.
If you do not have access to MacroModel version 5.5, you could use another implementation of MM2 or a related force field. These are likely to give different absolute values for the energy, which cannot be directly compared with those listed on these web pages. Promising structures will be reminimised using MacroModel, and recorded here, if they are lower in energy according to MacroModel. Details of the components of the energy for butane are available .