Appendix 2. A Deformation Density Refinement Program

A computer program is described that permits the refinement of the static charge deformation in a flexible atom‐centered multipole expansion, together with positional and vibrational parameters, against X‐ray and neutron data. Other options allow for rigid‐body constraints on some or all of the atomic vibration tensors and the imposition of local symmetry on a set of atoms such as a methyl group.