A procedure for the automatic choice of dynamical coordinates for semirigid molecules

A fully automatic procedure for the generation of constraint-conserving coordinates especially suited for the lattice dynamics of molecular crystals has been developed. Molecules with arbitrary constraints from rigid, semirigid to completely flexible may be treated. The procedure generates a mapping from new dynamical coordinates {ζ_p} to Cartesian atomic displacements. The new coordinates are defined by this mapping. It is accurate up to second order in ζ_p, thus allowing for the inclusion of force-constant contributions due to curved paths of atoms whose displacements are subject to some constraints. The procedure has been implemented in a lattice dynamics program currently under development. Exemplifying calculations on an oxalic acid molecule with several sets of constraints are presented.