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fftool

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Agilio Padua

This is a Python tool to build force field input files for molecular dynamics simulations of systems composed of molecules, ions or extended materials.

fftool creates initial files for classical, fixed-charge molecular dynamics simulations. A force field database ionic liquids is available in CL&P. For polarizable force field and simulations, check the CL&Pol tools and database.

Contents

Requirements

Obtaining

Download the files or clone the repository:

git clone https://github.com/agiliopadua/fftool.git

Tutorial

These are instructions on how to build an force field files and an initial configuration for a system composed of molecules, ions or materials.

  1. For each molecule, ion or fragment of a material prepare a file with atomic coordinates and eventually connectivity (covalent bonds). The formats accepted by this tool are .zmat, .xyz, .pdb or .mol, which are common formats in computational chemistry.

    A .zmat file has the molecule name in the first line, followed by one empty line, then the z-matrix. See the examples directory and the Wikipedia entry Z-matrix(chemistry). Variables can be used in place of distances, angles and dihedrals. fftool infers connectivity (topology) from the z-matrix by default. In this case cyclic molecules require additional connect records to close rings. Improper dihedrals can be indicated by improper records. If a reconnect record is present, then connectivity will be guessed based on bond distances from the force field (see below). Below the z-matrix and the informations above, the name of a file with force field parameters can be supplied.

    The XYZ file format .xyz contains atomic coordinates only. The name of a file with force field parameters can be given in the second line after the molecule name, and in this case connectivity is deduced from the bond lengths in the force field.

    The PDB file format .pdb is widely used for proteins. The name of a file with force field parameters can be given on a COMPND record after the molecule name. fftool infers connectivity from the bond lengths in the force field (CONECT records are not read).

    The MDL Molfile .mol file format contains a table with coordinates and also bonds. The name of a file with force field parameters can be given in the first line after the molecule name or in the third line. If the keyword reconnect is present after the force field filename, then connectivity will be deduced based on bond distances from the force field.

    There are many tools (Open Babel, Avogadro, VESTA) to create file in the above formats. Manual editing of the files is usually necessary in order to match the atom names with those of the force field.

  2. Use fftool to create an input file for packmol, which will use new _pack.xyz files with atomic coordinates for the components of your system. For help type fftool -h. For example, to build a simulation box with 40 ethanol and 300 water molecules and a density of 38.0 mol/L do:

     fftool 40 ethanol.zmat 300 spce.zmat -r 38.0

    Alternatively, the side length of the the simulation box (here cubic) can be supplied in angstroms:

     fftool 40 ethanol.zmat 300 spce.zmat -b 20.0

  3. Use packmol with the pack.inp file just created to generate the atomic coordinates in the simulation box:

     packmol < pack.inp

    A difficult convergence may indicate that density is too high, so adjust density or box size if necessary. For more complex spatial arrangements of molecules and materials you can modify the pack.inp to suit your needs (see the Packmol documentation). Atomic coordinates for the full system are written to simbox.xyz.

  4. Use fftool to create input files for LAMMPS (-l), OpenMM (-x), GROMACS (-g) ou DL_POLY (-d) containing the force field parameters and the coordinates of all the atoms (taken from simbox.xyz):

     fftool 40 ethanol.zmat 300 spce.zmat -r 38.0 -l

    If no force field information was given explicitly in the molecule files, a default LJ potential with parameters zeroed will be assigned to atoms. No terms for bonds, angles or torsions will be created. This is suitable when working with non-additive, bond-order or other potentials often used for materials. The input files for MD simulations will have to be edited manually to include interaction potentials.

Deducing Bonds and Angles

When inferring connectivity from atomic coordinates, distances in the coordinates file are compared with equilibrium distances specified for bonds in the force field, and a tolerance of +/-0.25 angstrom is used to decide if a bond is created. So, the bond lengths in the conformation present in the molecule file must be close to those in the force field specification for those bonds to be included in the potential energy fonction of the system. The user is advised to check the number of bonds by creating a test system with the minimum of molecules.

Angles will be assigned to groups of three atoms i-j-k, with i-j and j-k bonded, if the value of the angle in the conformation from the molecule file is within +/-15 degrees of the equilibrium angle in the force field. If not, even if the atoms i-j-k are bonded, their angle will not be present in the final potential energy function, although topologically the angle is there. When running fftool to create a force field file (with -l, -x, -g or -d option) a warning message will show which such topological angles have been "removed" because they deviate too much from the equilibrium angles in the force field. This removal of angles avoids problems with atoms that have more than four ligands, such as S or P atoms with five or six ligands. Around these centers there are topological angles of 180 degrees to which no potential energy of bending is attributed in force fields. For example, in the octahedral PF6- anion there are two different values of F-P-F angles: twelve 90 degree angles between adjacent F atoms, and three 180 degree angles between opposite F atoms; only the twelve 90 degree angles contribute with a harmonic potential energy function in most force fields.

The tolerances for bond distances and angle values, 0.25 angstrom and 15 degrees, respectively, were chosen based on judgement. They can be set by editing the fftool source, namely the global variables BondTol and AngleTol. Use with care because spurious bonds and angles may be created if the tolerances are set too large.

Improper Dihedrals

Improper dihedrals are often used to increase the rigidity of planar atoms (sp2) and differ from proper dihedrals in how they are defined. A proper dihedral i-j-k-l is defined between bonded atoms i-j, j-k, and k-l and corresponds to torsion around bond j-k, the dihedral being the angle between planes i-j-k and j-k-l. An improper dihedral i-j-k-l is defined between bonded atoms i-k, j-k and k-l, therefore k is a central atom bonded to the other three. fftool assumes the central atom of the improper dihedral to be the third in the list. Often in force fields the same potential energy function is used both for proper and improper torsions.

If improper records are supplied in a molecule file (in .zmat format) then those improper dihedrals are read by fftool. Otherwise, the script will search for candidate improper dihedrals on all atoms with three bonds, whatever the input format. Warning messages will be printed if there are atoms with three bonds, and these messages can be ignored if the atoms in question are not centers of improper torsions. The user is advised to check the number and order of the atoms in the true improper dihedrals in the files created, by testing with a minimal system.

Periodic Boundary Conditions

In molecular systems the initial configuration will generaly not contain molecules crossing boundaries of the simulation box. A buffer distance of 1.5 angstrom is reserved at the box boundaries to avoid overlap of molecules from periodic images in the initial configuration, as explained in the packmol documentation (this empty space is added by fftool only for orthogonal boxes). So the user should be aware of this empty volume when choosing the size of the box.

For simulations with extended materials it is possible to create chemical bonds across boundaries. Option -p allows specification of periodic conditions along x, y, z or combinations thereof. It is important in this case to supply box dimensions using the option -b <l> for a cubic box, -b <lx,ly,lz> for a general orthogonal box, or -b <a,b,c,alpha,beta,gamma> for a general parallelepiped (triclinic box). An energy minimization step prior to the start of the MD simulation is highly recommended because fftool will leave no extra space near the boundaries and certain molecules may overlap with those of neighboring images.

The coordinates of the atoms of the material have to be supplied in .xyz format and prepared carefully so that distances across periodic boundaries are within the tolerance to identify bonds. The user is advised to check the number of bonds in the output files created.

It is important that only the material for which bonds are to be established across boudaries is supplied in .xyz format. The initial files for other molecules in the system should be in .zmat or .mol formats, which contain connectivity information. This is to avoid spurious bonds between atoms of the molecular species that may happen to be positioned too close to boundaries.

The pack.inp file will likely need manual editing in order to position the atoms of the material precisely.

Force Field File Format

The fftool script reads a database of molecular force field parameters in xml format (similar to the format used by OpenMM), or in the original .ff format described below. See the examples directory.

The ff2xml script converts from the original to the xml format.

.ff format

Blank lines and lines starting with # are ignored.

There are five sections, with headings ATOMS, BONDS, ANGLES, DIHEDRALS and IMPROPER. Under each section heading, registers concerning the different types of term in the force field are given.

ATOMS records describe, for each type of atom:

BONDS records describe covalent bonds between intramolecular atom types:

ANGLES records describe valence angles between intramolecular atom types:

DIHEDRALS records describe torsion angles between intramolecular atom types:

IMPROPER records describe improper dihedral angles between intramolecular atom types:

References