This is the primary source of data, for both small molecules and for large molecules, giving the structure of a compound in the solid state. Positions of heavy atoms are located more accurately than lighter atoms. Hydrogen atom positions are frequently in calculated positions rather than observed positions. This method also gives the structure of the molecule in the solid state. This method is usually more accurate than X-ray crystallography and gives accurate positions for light atoms such as hydrogen.
Crystallographic data is the most freqently used starting point for molecular modelling.
It is often much easier to modify existing data for use than to start building using standard bond lengths, bond angles, and torsion angles. This method is capable of giving information about the solution phase structure. However this information is generally incomplete and needs extra information from other sources to give a complete picture. This method is becoming considerably more important in the last few years as a method for determining the structure of medium and large bio-molecules. There are several other less frequently used techniques, such as microwave spectroscopy, for determining molecular structure but these are not used routinely as starting information for molecular modelling.