MCCCS Towhee (Charmm27 Rigid Water)



    This section covers the Charmm27 Rigid Water force field as it is implemented into the towhee_ff_C27rigid file in the ForceFields directory. All of the Towhee atom types for this force field are listed, along with a short description of their meanings. This is a Lennard-Jones (12-6) force field and typically uses the Lorentz-Berthelot mixing rules. It uses the same parameters as the regular Charmm27 force field, with the notable exception that all of the intramolecular terms are rigid. Any discrepencies (especially typos) from the published force field values are the sole responsibility of Marcus G. Martin, and I welcome feedback on how this implementation compares with other programs.


References for Charmm27 Rigid Water
    The best literature references for the Charmm27 forcefield parameters are and More information about the Charmm27 forcefield is available on the Towhee Charmm27 web manual page. C27rigid is simply a rigid implementation of the Charmm27 water model where the equilibrium values of the bond and angle are set to be rigid instead of flexible. This force field was implemented to allow comparison with simulations using the standard molecular dynamics practice of freezing out intramolecular motion in water in order to speed the simuluations.
C27rigid in Towhee
    The official force field name for Charmm27 Rigid Water in Towhee is 'C27rigid'. Here I list all of the atom names for use in the towhee_input file, along with a brief description. Please note that the capitalization and spacing pattern is important and must be followed exactly as listed here.
    • 'HT' : Water hydrogen, modified TIP3P model
    • Oxygen
    • 'OT' : Water oxygen, modified TIP3P model
Coulombic Interactions
    Charmm27 Rigid Water utilizes point charges on atomic centers to represent the charge distribution on a molecule. The 'bond increment' method for assigning charges is implemented for this force field. Please see the inpstyle 2 documentation for further information on enabling this option. Here I list the bond increments for the atom names listed above. The value of the bond increment is added to the first atom listed and subtracted from the second atom listed.
    • HT - OT: 0.417


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Send comments to: Marcus G. Martin
Last updated: September 07, 2005