Molecular orientation requirements

adf requires that the molecule has a specific orientation in space, as follows:

• The origin is a fixed point of the symmetry group.
• The z-axis is the main rotation axis, xy is the σ_h-plane (axial groups, C(s)).
• The x-axis is a C₂ axis (D symmetries).
• The xz-plane is a σ_v-plane (C_nv symmetries).
• In T_d and O_h the z-axis is a fourfold axis (S₄ and C₄, respectively) and the (111)-direction is a threefold axis.

If the user-specified symmetry equals the true symmetry of the nuclear frame (including electric field and point charges) the program will adapt the input coordinates to the above requirements, if necessary. If no symmetry has been specified at all adf assumes you have specified the symmetry of the nuclear frame, accounting for any fields. If a subgroup has been specified for the molecular symmetry the input coordinates will be used as specified by the user. If a Z-matrix input is given this implies for the Cartesian coordinates: first atom in the origin, second atom on the positive x-axis, third atom in the xy-plane with positive y value.