Skip to content

X–H bond length

The xh_bond_length quantity is a small provisional addition in the current release line.

Its purpose is not to claim a complete literature survey of X–H bond lengths. Instead, it provides a stable, provenance-aware starting point for hydrogen-normalisation workflows and related geometry code.

Packaged target dataset

CSD legacy X–H neutron-normalisation targets (csd_legacy_xh_cno)

  • What it is: the fixed C–H, N–H, and O–H target lengths used by ConQuest for terminal-hydrogen normalisation.
  • Coverage: only parent elements C, N, and O.
  • Values: C–H = 1.089 Å, N–H = 1.015 Å, O–H = 0.993 Å.
  • Primary provenance: the ConQuest user guide section Hydrogen Atom Location in Crystal Structure Analyses.
  • Secondary provenance: Allen & Bruno (2010), which the ConQuest guide cites for these defaults.

How atomref uses it

The built-in DEFAULT_XH_POLICY treats csd_legacy_xh_cno as a sparse target set and restores missing parent elements through a fitted linear transfer from cordero2008 covalent radii.

That means the package draws a sharp line between:

  • curated dataset values — currently only C, N, and O, and
  • policy-generated values — inferred for other parent elements when the predictor policy can supply a covalent radius.

Scope note

This is intentionally a small addendum rather than full X–H support. Broader X–H datasets, richer policies, and more complete literature treatment are planned for 0.2.x.