7 The *static* properties (see :numref:`Table %s <tab-statprop>`) assigned to the atom
8 types are assigned based on data in several places. The mass is listed
9 in ``atomtypes.atp`` (see :ref:`atomtype`), whereas the charge is listed
10 in :ref:`rtp` (:ref:`rtp` = **r**\ esidue **t**\ opology **p**\ arameter file,
11 see :ref:`rtp`). This implies that the charges are only defined in the
12 building blocks of amino acids, nucleic acids or otherwise, as defined
13 by the user. When generating a :ref:`topology <top>` using the :ref:`pdb2gmx <gmx pdb2gmx>`
14 program, the information from these files is combined.
18 .. table:: Static atom type properties in |Gromacs|
20 +----------+------------------+----------+
21 | Property | Symbol | Unit |
22 +==========+==================+==========+
24 +----------+------------------+----------+
26 +----------+------------------+----------+
27 | Charge | q | electron |
28 +----------+------------------+----------+
29 | epsilon | :math:`\epsilon` | kJ/mol |
30 +----------+------------------+----------+
31 | sigma | :math:`\sigma` | nm |
32 +----------+------------------+----------+
40 The non-bonded parameters consist of the van der Waals parameters V (``c6``
41 or :math:`\sigma`, depending on the combination rule) and W (``c12`` or
42 :math:`\epsilon`), as listed in the file ``ffnonbonded.itp``, where ``ptype`` is
43 the particle type (see :numref:`Table %s <tab-ptype>`). As with the bonded
44 parameters, entries in ``[ *type ]`` directives are applied to their counterparts in
45 the topology file. Missing parameters generate warnings, except as noted
46 below in section :ref:`pairinteractions`.
51 ;name at.num mass charge ptype V(c6) W(c12)
52 O 8 15.99940 0.000 A 0.22617E-02 0.74158E-06
53 OM 8 15.99940 0.000 A 0.22617E-02 0.74158E-06
57 ; i j func V(c6) W(c12)
58 O O 1 0.22617E-02 0.74158E-06
59 O OA 1 0.22617E-02 0.13807E-05
62 **Note** that most of the included force fields also include the ``at.num.``
63 column, but this same information is implied in the OPLS-AA ``bond_type``
64 column. The interpretation of the parameters V and W depends on the
65 combination rule that was chosen in the ``[ defaults ]`` section of the topology file
71 \mbox{for combination rule 1}: & &
73 \mbox{V}_{ii} & = & C^{(6)}_{i} & = & 4\,\epsilon_i\sigma_i^{6} &
74 \mbox{[ kJ mol$^{-1}$ nm$^{6}$ ]}\\
75 \mbox{W}_{ii} & = & C^{(12)}_{i} & = & 4\,\epsilon_i\sigma_i^{12} &
76 \mbox{[ kJ mol$^{-1}$ nm$^{12}$ ]}\\
79 \mbox{for combination rules 2 and 3}: & &
81 \mbox{V}_{ii} & = & \sigma_i & \mbox{[ nm ]} \\
82 \mbox{W}_{ii} & = & \epsilon_i & \mbox{[ kJ mol$^{-1}$ ]}
83 \end{array}\end{aligned}
85 Some or all combinations for different atom types can be given in the
86 ``[ nonbond_params ]`` section, again with parameters V and
87 W as defined above. Any combination that is not given will be computed
88 from the parameters for the corresponding atom types, according to the
94 \mbox{for combination rules 1 and 3}: & &
96 C^{(6)}_{ij} & = & \left(C^{(6)}_i\,C^{(6)}_j\right)^{\frac{1}{2}} \\
97 C^{(12)}_{ij} & = & \left(C^{(12)}_i\,C^{(12)}_j\right)^{\frac{1}{2}}
100 \mbox{for combination rule 2}: & &
102 \sigma_{ij} & = & \frac{1}{2}(\sigma_i+\sigma_j) \\
103 \epsilon_{ij} & = & \sqrt{\epsilon_i\,\epsilon_j}
104 \end{array}\end{aligned}
106 When :math:`\sigma` and :math:`\epsilon` need to be supplied (rules 2
107 and 3), it would seem it is impossible to have a non-zero :math:`C^{12}`
108 combined with a zero :math:`C^6` parameter. However, providing a
109 negative :math:`\sigma` will do exactly that, such that :math:`C^6` is
110 set to zero and :math:`C^{12}` is calculated normally. This situation
111 represents a special case in reading the value of :math:`\sigma`, and
114 There is only one set of combination rules for Buckingham potentials:
119 A_{ij} &=& \left(A_{ii} \, A_{jj}\right)^{1/2} \\
120 B_{ij} &=& 2 / \left(\frac{1}{B_{ii}} + \frac{1}{B_{jj}}\right) \\
121 C_{ij} &=& \left(C_{ii} \, C_{jj}\right)^{1/2}
129 (*i.e.* bonds, bond angles, improper and proper dihedrals) are listed in
130 ``ffbonded.itp``. The entries in this database describe,
131 respectively, the atom types in the interactions, the type of the
132 interaction, and the parameters associated with that interaction. These
133 parameters are then read by
134 :ref:`grompp <gmx grompp>` when processing a
135 topology and applied to the relevant bonded parameters, *i.e.*
136 ``bondtypes`` are applied to entries in the
137 ``[ bonds ]`` directive, etc. Any bonded parameter that is
138 missing from the relevant :``[ *type ]`` directive generates
139 a fatal error. The types of interactions are listed in
140 :numref:`Table %s <tab-topfile2>`. Example excerpts from such files
147 C O 1 0.12300 502080.
148 C OM 1 0.12500 418400.
153 HO OA C 1 109.500 397.480
154 HO OA CH1 1 109.500 397.480
159 NR5* NR5 2 0.000 167.360
160 NR5* NR5* 2 0.000 167.360
164 ; j k func phi0 cp mult
165 C OA 1 180.000 16.736 2
166 C N 1 180.000 33.472 2
171 ; Ryckaert-Bellemans Dihedrals
174 CP2 CP2 3 9.2789 12.156 -13.120 -3.0597 26.240 -31.495
176 In the ``ffbonded.itp`` file, you can add bonded parameters.
177 If you want to include parameters for new atom types, make sure you
178 define them in ``atomtypes.atp`` as well.
180 For most interaction types, bonded parameters are searched and assigned
181 using an exact match for all type names and allowing only a single set
182 of parameters. The exception to this rule are
185 ``[ dihedraltypes ]`` wildcard atom type names can be
186 specified with the letter ``X`` in one or more of the four
187 positions. Thus one can for example assign proper dihedral parameters
188 based on the types of the middle two atoms. The parameters for the entry
189 with the most exact matches, i.e. the least wildcard matches, will be
190 used. Note that |Gromacs| versions older than 5.1.3 used the first match,
191 which means that a full match would be ignored if it is preceded by an
192 entry that matches on wildcards. Thus it is suggested to put wildcard
193 entries at the end, in case someone might use a forcefield with older
194 versions of |Gromacs|. In addition there is a dihedral type 9 which adds
195 the possibility of assigning multiple dihedral potentials, useful for
196 combining terms with different multiplicities. The different dihedral
197 potential parameter sets should be on directly adjacent lines in the
198 ``[ dihedraltypes ]`` section.