This section covers an extension to the Weiner et al. 1986 force field.
Unlike most force fields, Dick and Ritchie 1994 is only useful in combination with the
Weiner et al. 1986 force field.
All of the additional Towhee atom types for the Dick and Ritchie 1994 extension are listed
, along with a short description of their meanings. For
more information about the Amber family of force fields see the Amber web site.
Note that Dick and Ritchie 1994 is a 12-6 plus 12-10 H-bond force field and can only be combined with
other '12-6 plus 12-10 H-bond' force fields.
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 Dick and Ritchie 1994
Dick and Ritchie 1994 in Towhee
The official force field name for Dick and Ritchie 1996 in Towhee is 'Weiner1986'. This indicates that
Dick and Ritchie 1994 is not a stand alone force field and is only useful in combination with the
Weiner et al. 1986 force field. In order to use the Dick and Ritchie 1994
extensions you need to include the towhee_ff_Dick1994 file from the ForceFields directory. Here I list all of
the atom names for use in the towhee_input file, along with a brief description taken
from Dick and Ritchie 1994. These atom types are designed to
be mixed freely with the atom types described in the Weiner et al. 1986 documentation.
Please note that the capitalization and spacing pattern is important
and must be followed exactly as listed here.
- Nitrogen (atomic)
- 'NN' : sp2 nitrogen in a nitro group.
Point charges for pentaerythritol tetranitrate (PETN) are provided in Table III of the Dick
and Ritchie 1994 paper. However, the charges listed there add up to -0.003 for the PETN molecule so there must be some sort
of typo in that table as the molecule should be net neutral. They also list different charges for the two symmetric oxygens on the
nitro group, which might make sense for crystals, but does not make sense for fluids. Here I list their suggested charges for PETN
and also the modifications I am currently using.
- Atom: Dick and Ritchie 1996 charges [Martin unpublished charges]
- CT (central carbon): -0.479 [-0.476]
- CT (branch carbon): -0.030 [-0.030]
- OS (ether-like oxygen): -0.476 [-0.476]
- NN (nitro nitrogen): 0.771 [0.771]
- O (nitro oxygens): -0.346 and -0.388 [-0.367]
Improper torsions are not automatically generated by the Towhee code as the rules for determining where they are applied
are not always straight-forward. Dick and Ritchie do not specify any additional improper torsion parameters so I do not
believe any apply to the PETN molecule. For more information about improper torsions on other molecules please consult
the Weiner et al. 1986 web manual.
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