Roles of starting geometries in quantum mechanics studies of cellobiose

In earlier work, a relaxed HF/6-31G(d) energy surface was constructed for the fraction of φ, ψ space that contains most geometries from crystals of molecules similar to cellobiose. Two regions around other minima were examined with unconstrained B3LYP/6-31 + G(d) minimisations, as were two sub-regions covered by the map. More than 100 different exo-cyclic group arrangements (“starting geometries”), selected for stability and low energies, were tested at each φ, ψ point and in the four sets of unconstrained minimisations. The influence of these starting geometries was studied in the present work. Twenty-four unique structures gave the lowest energy at one or more of the 81 φ, ψ grid points. Structures from the unconstrained minimisations covered wide ranges of φ, ψ space. Also, the 11–18 kcal/mol ranges of relative energies for the unconstrained minimisations resulting from the different starting geometries were comparable to the 20 kcal/mol range of energies on the relaxed map where some conformations were held in high-energy φ, ψ shapes by the mapping procedure. Thus, exo-cyclic group orientations and the φ and ψ torsion angles are both important factors in determining the likely structures and energies of a disaccharide.