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MCCCS Towhee (Catlow/Faux)

 

 

Overview
    This section covers the Catlow and Faux force field as it is implemented into the towhee_ff_CatlowFaux file in the ForceFields directory. All of the Towhee atom types for the Catlow and Faux force field are listed, along with a short description of their meanings. Note that the Catlow and Faux force fields are a hybrid of Lennard-Jones (12-6) and an exponential replusion and cannot be combined with any other force fields. I would like to acknowledge David A. Faux for providing very useful guidance about implementing Catlow and Faux force field. 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 Catlow and Faux
    This force field comes from a variety of sources. Several authors have used zeolite parameters that are similar (but often slightly different) to those published in The implementation in Towhee uses the zeolite (OZ, Al, Si, Na) parameters as modified in with the exception that there is a typo in Table 1 of that paper which listed harmonic terms for the Al-OZ-Si and Si-OZ-Al angles. That should instead be harmonic parameters for the OZ-Al-OZ and OZ-Si-OZ angles. Note that these harmonic terms are in addition to the normal van der Waals and coulombic interaction between these atoms. There is no additional harmonic term for either the Al-OZ-Si or Si-OZ-Al (personal communication between M.G. Martin and D.A. Faux February 13, 2001).
    The hydrocarbon (Cc and Hc) nonbonded parameters, and the cross terms with OZ, Al, Si (note the cross terms are set to zero for Cc-Si, Cc-Al, Hc-Si, and Hc-Al) are from However, we wanted a fully flexible alkane model so we used the bonded parameters from instead of the semi-rigid model from Raj et al. 1999.
    I also concocted some cross terms for Na-Cc and Na-Hc by combining Raj et al. 1999 with sodium parameters from using geometric mixing rules.
Catlow/Faux in Towhee
    The official force field name for these parameters is 'CatlowFaux' (starting with version 3.2.9) Here I list all of the Catlow and Faux atom names for use in the towhee_input file, along with a brief description of what atoms we are talking about. Please note that the capitalization and spacing pattern is important and must be followed exactly as listed here.
    • 'Cc' : carbon in an alkane
    • 'Hc' : hydrogen bonded to carbon in an alkane
    • 'Al' : aluminum in a zeolite
    • 'Na' : sodium ion
    • 'OZ' : oxygen in a zeolite
    • 'Si' : silicon in a zeolite
Coulombic Interactions
    This force field uses point charges on atomic centers. Here we list suggested charges for each of the atom types in this force field.
    Catlow charges are only listed for n-octane in Raj et al. 1999. It is not obvious how to interpolate those charges for other alkanes, but it would be consistent with the reasoning in that paper to set all of the Hydrogens to 0.100 and then evenly divide the counter charge across all of the carbons.
    • 'Cc' : In n-octane this is set to -0.225. One possible way to do this for general alkanes would be a charge equal to the number of hydrogens times the charge on each hydrogen divided by the total number of carbons.
    • 'Hc' : In n-octane this is set to 0.100. One possible way to do this for general alkanes would be to always set the Hc charge to 0.100 and adjust the counter charge on the carbons.
    Faux charges from Faux et al. 1997
    • 'Al' : 2.775
    • 'Na' : 1.0
    • 'OZ' : -1.86875
    • 'Si' : 3.7
Improper torsions
    There are no improper torsions in the Catlow or Faux force fields.
Proteins
    The Catlow and Faux force fields are designed for zeolites and are not suited for proteins.
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Send comments to: Marcus G. Martin
Last updated: September 07, 2005