Three sets of calculations were performed:
The total energy of the molecule was calculated for a number of different values of D2 and D3 on a 60 degree grid over half of the phase plane defined by these angles. All of the ions remained static during these calculations.
The total energy of the molecule was recalculated for a number of points on this map while the structure of the molecule was allowed to relax into its minimum energy conformation. The dihedral angles defining the point were constrained to remain constant during this calculation.
A minimum energy conformation was found by performing an unconstrained relaxation to allow the molecule to attain its ground state.