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MCCCS Towhee: Morrow and Maginn 2002
Overview
    This section covers the Morrow and Maginn 2002 (Morrow2002) force field as it is implemented into the towhee_ff_Morrow2002 file in the ForceFields directory. This force field was designed for an ionic liquid of 1-n-butyl-3-methylimidazolium [bmim] hexafluorophosphate [PF6]. Note that this is a Lennard-Jones force field that uses the 'Lorentz-Berthelot' mixing rules. Any discrepencies (especially typos) from the published values are the sole responsibility of Marcus Martin, and I welcome feedback on how this implementation compares with other programs.
References for Morrow and Maginn 2002 Typos and comments for Morrow and Maginn 2002
  • The parameters implemented for this force field are from the erratum.
  • All bonds involving hydrogen were held rigid in their work using SHAKE, and are therefore implemented here as rigid bonds.
  • Full Lennard-Jones interactions are used for 1-4 interactions, while the Coulombic 1-4 terms are scaled by 1/2 [personal communication between E.J. Maginn and M.G. Martin 04-21-2006].
  • The second to last angle term in Table 2 of the erratum should read N-C7-H7,8 instead of N-C7-H8,9 [personal communication between E.J. Maginn and M.G. Martin 04-21-2006].
  • No value for the H-C8-C9-H dihedral was specified in their papers. It has been implemented here using the published H-C7-C8-H term.
  • The authors claim that the charge on PF6 adds up to -0.904, and the charge on bmim adds up to 0.904. However, the partial charges they list for PF6 using assymmetric charge assignments adds up to -0.903, which does not properly balance the charge of the bmin. I have adjusted this by utlizing the stated charges on the Fluorines, and placing the remainder of the charge on the Phosphorous.
Morrow and Maginn 2002 in Towhee
    The official force field name for Morrow and Maginn 2002 in Towhee is 'Morrow2002'. This list contains atom names for use in the towhee_input file, along with a brief description. These names correspond to those used in Morrow and Maginn 2002. Please note that the capitalization and spacing pattern is important and must be followed exactly as listed here.
      'N1' : Ring Nitrogen bonded to C2, C5, and C7 in bmim
      'C2' : Ring Carbon bonded to N1, N3, and H1 in bmim
      'N3' : Ring Nitrogen bonded to C2, C4, and C6 in bmim
      'C4' : Ring Carbon bonded to N3, C5, and H2 in bmim
      'C5' : Ring Carbon bonded to N1, C4, and H3 in bmim
      'C6' : methyl Carbon bonded to N3, H4, H5, and H6 in bmim
      'C7' : methylene Carbon bonded to N1, C8, H7, and H8 in bmim
      'C8' : methylene Carbon bonded to C7, C9, H9, and H10 in bmim
      'C9' : methylene Carbon bonded to C8, C10, H11, and H12 in bmim
      'C10' : methyl Carbon bonded to C9, H13, H14, and H15 in bmim
      'H1' : Hydrogen bonded to C2 in bmim
      'H2' : Hydrogen bonded to C4 in bmim
      'H3' : Hydrogen bonded to C5 in bmim
      'H4' : Hydrogen bonded to C6 in bmim
      'H5' : Hydrogen bonded to C6 in bmim
      'H6' : Hydrogen bonded to C6 in bmim
      'H7' : Hydrogen bonded to C7 in bmim
      'H8' : Hydrogen bonded to C7 in bmim
      'H9' : Hydrogen bonded to C8 in bmim
      'H10' : Hydrogen bonded to C8 in bmim
      'H11' : Hydrogen bonded to C9 in bmim
      'H12' : Hydrogen bonded to C9 in bmim
      'H13' : Hydrogen bonded to C10 in bmim
      'H14' : Hydrogen bonded to C10 in bmim
      'H15' : Hydrogen bonded to C10 in bmim
      'P' : P in hexafluorophosphate
      'F' : generic F in hexafluorophosphate if you want to use the symmetric charge distribution version of that molecule.
      'F1' : specific F in hexafluorophosphate if you want to use the assymmetric charge distribution version of that molecule.
      'F2' : specific F in hexafluorophosphate if you want to use the assymmetric charge distribution version of that molecule.
      'F3' : specific F in hexafluorophosphate if you want to use the assymmetric charge distribution version of that molecule.
      'F4' : specific F in hexafluorophosphate if you want to use the assymmetric charge distribution version of that molecule.
      'F5' : specific F in hexafluorophosphate if you want to use the assymmetric charge distribution version of that molecule.
      'F6' : specific F in hexafluorophosphate if you want to use the assymmetric charge distribution version of that molecule.
Coulombic interactions
    This force field uses point charges and has been set up to assign the point charges using the bond increment method. The charges for bmim are assigned according to Table 1 of Morrow and Maginn 2002. There are two possible charge distributions for hexafluorophosphate. If you specify each of the Fluorine atoms (F1, ..., F6) then the charges are assigned according to Table 1 of Morrow and Maginn 2002, with the exception that the P charge is 1.457 instead of the stated 1.478 (see the notes above). If you specify a generic Fluorine atom (F), then the charge on the Phosphorous is the same as in Table 1, while the Fluorines are assigned symmetric charges in order to achieve a total molecule charge of -0.904.
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Last updated: July 22, 2021 Send comments to: Marcus Martin