Platon/squeeze ton Spek Bijvoet Center

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  • Ton Spek

  • Bijvoet Center

  • Utrecht University,

  • The Netherlands.

  • PLATON Workshop

  • Chicago, 24-July-2010

The Disordered Solvent Problem

  • Molecules of interest often co-crystallize (only) with the inclusion of a suitable solvent molecule.

  • Solvent molecules often fill voids in a structure with little interaction and are often located on symmetry sites and with population less than 1.0

  • Sometimes even the nature of the (mixture) of included solvent(s) is unclear.

  • Inclusion of the scattering contribution of the solvent to the structure factors can be done either with an (elaborate) disorder model or with the SQUEEZE approach.

Automated Detection of Solvent Accessible Voids

  • A typical crystal structure has only in the order of 65% of the available space filled.

  • The remainder volume is in voids (cusps) in-between atoms (too small to accommodate an H-atom)

  • Solvent accessible voids can be defined as regions in the structure that can accommodate at least a sphere with radius 1.2 Angstrom without intersecting with any of the van der Waals spheres assigned to each atom in the structure.

  • Next Slide: Void Algorithm: Cartoon Style 


  • Move a probe with radius 1.2 Ang over a fine (0.2 Angstrom) grid through the unit cell.

  • Start a new void when a grid point is found that is at least 1.2 Angstrom outside the van der Waals surface of all atoms.

  • Expand this void with connected grid points with the same property until completed.

  • Find new starting grid point for the next void until completion.

  • Expand the ‘Ohashi’ volumes with grid points within 1.2 Angstrom to surface grid points.


  • Detection of (possibly missed) Solvent Accessible Voids in a Structure

  • Calculation of the Kitaigorodskii Packing Index

  • Determination of the available space in solid state reactions (Ohashi)

  • Determination of pore volumes, pore shapes and migration paths in micro-porous crystals

  • As part of the SQUEEZE routine to handle the contribution of disordered solvents in a crystal structure refinement.


  • Takes the contribution of disordered solvents to the calculated structure factors into account by back-Fourier transformation of density found in the ‘solvent accessible volume’ outside the ordered part of the structure (iterated).

  • Two Options:

  • Refine with SHELXL using the solvent free .hkl

  • Or use CRYSTALS using the SQUEEZE solvent contribution to F(calc) and the full F(obs).

  • Note:SHELXL lacks option for fixed contribution to Structure Factor Calculation.

SQUEEZE In the Complex Plane

SQUEEZE Algorithm

  • Calculate difference Fourier map (FFT)

  • Use the VOID-map as a mask on the FFT-map to set all density outside the VOID’s to zero.

  • FFT-1 this masked Difference map -> contribution of the disordered solvent to the structure factors

  • Calculate an improved difference map with F(obs) phases based on F(calc) including the recovered solvent contribution and F(calc) without the solvent contribution.

  • Recycle to 2 until convergence.

Test Example with Calculated Data

  • ‘Observed Data’ were calculated from the published coordinates.

  • The ether molecule was subsequently removed

  • SQUEEZE was tested to see whether the method recovers the ether contribution to the structure factors.

Recommended SQUEEZE Procedure in combination with SHELXL97

  • Refine a discrete atom model including Hydrogen atoms with SHELXL => .res

  • Delete (when applicable) all ‘atoms’ used to tentatively model the disordered region => .res

  • Do a PLATON/SQUEEZE run with .res (from 2) and .hkl (from 1)

  • Copy .res => .ins and .hkp => .hkl in a new directory

  • Refine with SHELXL (with .ins and .hkl from 4)

  • Analyze results and optionally repeat from 3 (with .res from 5 and original .hkl)

  • Do final ‘CALC FCF’ with PLATON to get proper .fcf (with .res and .hkl from 5 in a new directory) => .hkp

  • Rename .hkp => .fcf

  • Append the SQUEEZE info in .sqf to the .cif from 5

Real World Example

  • THF molecule disordered over a center of inversion

  • Comparison of the result of a disorder model refinement with a SQUEEZE refinement


  • A Complete data set, including non-zero intensity low order reflections for the estimation of the number of electrons in the void region.

  • No significant residual unresolved density excursions in the difference map for the ordered part of the structure

Limitations of SQUEEZE

  • SQUEEZE can currently not handle properly most cases of main molecule disorder that is coupled with the solvent disorder.

  • SQUEEZE is currently incompatible with twinning

  • SQUEEZE needs a sufficient data resolution to give meaningful results

Concluding Remarks

  • The CSD includes now in the order of 1000 entries where SQUEEZE was used.

  • Care should be taken with issues such as charge balance that effects the chemistry involved.

  • The use of the SQUEEZE procedure should be detailed in the experimental section of a paper based on its use.

Additional Info


  • The Bypass Paper:

  • P. van der Sluis & A.L.Spek (1990).Acta Cryst., A46, 194-201.

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