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
68 .. math:: \begin{aligned}
69 \mbox{for combination rule 1}: & &
71 \mbox{V}_{ii} & = & C^{(6)}_{i} & = & 4\,\epsilon_i\sigma_i^{6} &
72 \mbox{[ kJ mol$^{-1}$ nm$^{6}$ ]}\\
73 \mbox{W}_{ii} & = & C^{(12)}_{i} & = & 4\,\epsilon_i\sigma_i^{12} &
74 \mbox{[ kJ mol$^{-1}$ nm$^{12}$ ]}\\
77 \mbox{for combination rules 2 and 3}: & &
79 \mbox{V}_{ii} & = & \sigma_i & \mbox{[ nm ]} \\
80 \mbox{W}_{ii} & = & \epsilon_i & \mbox{[ kJ mol$^{-1}$ ]}
81 \end{array}\end{aligned}
82 :label: eqndefcombrule
84 Some or all combinations for different atom types can be given in the
85 ``[ nonbond_params ]`` section, again with parameters V and
86 W as defined above. Any combination that is not given will be computed
87 from the parameters for the corresponding atom types, according to the
90 .. math:: \begin{aligned}
91 \mbox{for combination rules 1 and 3}: & &
93 C^{(6)}_{ij} & = & \left(C^{(6)}_i\,C^{(6)}_j\right)^{\frac{1}{2}} \\
94 C^{(12)}_{ij} & = & \left(C^{(12)}_i\,C^{(12)}_j\right)^{\frac{1}{2}}
97 \mbox{for combination rule 2}: & &
99 \sigma_{ij} & = & \frac{1}{2}(\sigma_i+\sigma_j) \\
100 \epsilon_{ij} & = & \sqrt{\epsilon_i\,\epsilon_j}
101 \end{array}\end{aligned}
102 :label: eqngivencombrule
104 When :math:`\sigma` and :math:`\epsilon` need to be supplied (rules 2
105 and 3), it would seem it is impossible to have a non-zero :math:`C^{12}`
106 combined with a zero :math:`C^6` parameter. However, providing a
107 negative :math:`\sigma` will do exactly that, such that :math:`C^6` is
108 set to zero and :math:`C^{12}` is calculated normally. This situation
109 represents a special case in reading the value of :math:`\sigma`, and
112 There is only one set of combination rules for Buckingham potentials:
114 .. math:: \begin{array}{rcl}
115 A_{ij} &=& \left(A_{ii} \, A_{jj}\right)^{1/2} \\
116 B_{ij} &=& 2 / \left(\frac{1}{B_{ii}} + \frac{1}{B_{jj}}\right) \\
117 C_{ij} &=& \left(C_{ii} \, C_{jj}\right)^{1/2}
119 :label: eqnbuckinghamcombrule
126 (*i.e.* bonds, bond angles, improper and proper dihedrals) are listed in
127 ``ffbonded.itp``. The entries in this database describe,
128 respectively, the atom types in the interactions, the type of the
129 interaction, and the parameters associated with that interaction. These
130 parameters are then read by
131 :ref:`grompp <gmx grompp>` when processing a
132 topology and applied to the relevant bonded parameters, *i.e.*
133 ``bondtypes`` are applied to entries in the
134 ``[ bonds ]`` directive, etc. Any bonded parameter that is
135 missing from the relevant :``[ *type ]`` directive generates
136 a fatal error. The types of interactions are listed in
137 :numref:`Table %s <tab-topfile2>`. Example excerpts from such files
144 C O 1 0.12300 502080.
145 C OM 1 0.12500 418400.
150 HO OA C 1 109.500 397.480
151 HO OA CH1 1 109.500 397.480
156 NR5* NR5 2 0.000 167.360
157 NR5* NR5* 2 0.000 167.360
161 ; j k func phi0 cp mult
162 C OA 1 180.000 16.736 2
163 C N 1 180.000 33.472 2
168 ; Ryckaert-Bellemans Dihedrals
171 CP2 CP2 3 9.2789 12.156 -13.120 -3.0597 26.240 -31.495
173 In the ``ffbonded.itp`` file, you can add bonded parameters.
174 If you want to include parameters for new atom types, make sure you
175 define them in ``atomtypes.atp`` as well.
177 For most interaction types, bonded parameters are searched and assigned
178 using an exact match for all type names and allowing only a single set
179 of parameters. The exception to this rule are
182 ``[ dihedraltypes ]`` wildcard atom type names can be
183 specified with the letter ``X`` in one or more of the four
184 positions. Thus one can for example assign proper dihedral parameters
185 based on the types of the middle two atoms. The parameters for the entry
186 with the most exact matches, i.e. the least wildcard matches, will be
187 used. Note that |Gromacs| versions older than 5.1.3 used the first match,
188 which means that a full match would be ignored if it is preceded by an
189 entry that matches on wildcards. Thus it is suggested to put wildcard
190 entries at the end, in case someone might use a forcefield with older
191 versions of |Gromacs|. In addition there is a dihedral type 9 which adds
192 the possibility of assigning multiple dihedral potentials, useful for
193 combining terms with different multiplicities. The different dihedral
194 potential parameter sets should be on directly adjacent lines in the
195 ``[ dihedraltypes ]`` section.