This example is a simple simulation of many long alkane chains (hexadecane) in a box near the boiling point at atmospheric pressure. The hexadecane molecule in this example (defined in the hexadecane.lt file) was constructed from monomeric subunits (named "CH2", and "CH3"). Note: This particular example uses the a variant of the OPLSAA force-field suitable for long alkane chains (sometimes called the "LOPLSAA" force field).
The number of molecules and simulation box size can be controlled by editing the system.lt file. The length of each polymer can be controlled by editing the hexadecane.lt file. The simulation contitions can be controlled by editing the run.in.npt file.
If this is your first time learning how to build a polymer in moltemplate, I suggest starting with the butane example instead.
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To build the files which LAMMPS needs, follow the instructions in: README_setup.sh
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To run LAMMPS with these files, follow these instructions: README_run.sh
(The instructions in "README_remove_irrelevant_info.sh" are optional. (If you notice a problem with this example, please report it.)
The "Hexadecane" molecule, as well as the "CH2", and "CH3" monomers it contains, use the LOPLSAA force-field. As with all of the OPLSAA examples, when we define these molecules, we only specify the atom names, bond list, and coordinates. We do not have to list the atom charges, angles, dihedrals, or impropers. The rules for creating atomic charge and angle topology are contained in the "loplsaa.lt" and "oplsaa.lt" files. To let moltemplate know that you want to use these rules, define your molecules (and molecular subunits) this way:
import "loplsaa.lt"
CH2 inherits OPLSAA { ... } # (see "ch2group.lt")
CH3 inherits OPLSAA { ... } # (see "ch3group.lt")
Hexadecane inherits OPLSAA { ... } # (see "hexadecane.lt")
There are only a few differences between LOPLSAA and OPLSAA. The LOPLSAA force field contains a few extra atom types and dihedral interactions which improve the accuracy of long alkane chains. The "loplsaa.lt" file (referenced above) incorporates these extra atom and dihedral types in the existing OPLSAA force field instead of creating a new force field named "LOPLSAA". (There is no separate "LOPLSAA" force field object. I apologize if this is confusing.) You can mix LOPLSAA and OPLSAA atoms in the same molecule.
In this example, atomic charge for OPLSAA atoms is determined by @atom type (...according to a lookup table located at the beginning of the "oplsaa.lt" file). (Any atomic charges listed in the "Data Atoms" section of your molecules' LT files will be ignored.) These charges can be overridden. See here for instructions explaining how to customize atomic charge.
If necessary, you can customize existing bonds, angles, dihedrals etc. in your molecule (eg. Hexadecane), or add new ones (if the force field does not define them). To do this, edit the corresponding LT file (eg. "hexadecane.lt"), and add extra sections to that file (eg. write("Data Bonds") or write("Data Angles")). Then add a list of bonded interactions to these sections (containing lines similar to "$bond:c7h5 @bond:CustomType $atom:c7 $atom:h5"). By default, this will override the bond and bonded angular interactions created by the force field. For more details, read the chapter in the moltemplate manual named "Customizing molecule position and topology".)