molgri 1.0.3

Generate molecular pseudotrajectories based on rotational grids.

molecularRotationalGrids

The python package molgri has three main purposes: 1) generation of rotation grids, 2) analysis of said grids and 3) generation of pseudotrajectories (PTs). PTs are .gro files with several timesteps in which the interaction space of two molecules is systematically explored. We provide user-friendly, one-line scripts for rotation grid generation and analysis as well as pseudotrajectory generation and give instructions how to use PTS with external tools like VMD and GROMACS for further analysis.

Below, we show examples of rotation grids created with different algorithms and figures of a protein-ion pseudotrajectory as well as some analysis plots. All plots and animations are created directly with the molgri package, except the PT plot where the output of molgri is drawn using VMD.

Installation

This is a python library that can be easily installed using:

pip install molgri

Running examples

To explore the capabilities of the package, the user is encouraged to run the following example commands (the commands should all be executed in the same directory, we recommend an initially empty directory).

molgri-io --examples
molgri-grid -N 250 -algo ico --draw --animate --animate_ordering --statistics
molgri-pt -m1 H2O -m2 NH3 -rotgrid cube3D_15 -transgrid "range(1, 5, 2)"

****The first-line command molgri-io creates the 📂 input/ and 📂 output/ folder structure. This command should be run in each new directory before running other commands. The optional flag --examples provides some sample input files that we will use later.

The second command molgri-grid is used to generate rotation grids. It is necessary to specify the number of grid points -N and the algorithm -algo (select from: systemE, randomE, randomQ, cube4D, cube3D, ico; we recommend ico). Other flags describe optional figures and animations to save. All generated files can be found in the output/ folder.

The last command molgri-pt creates a pseudotrajectory. This scripts needs two file inputs that should be provided in input/. Both must be .gro files, each containing a single molecule. Due to the flag -m1 H2O the program will look for a file input/H2O.gro and use it as a fixed molecule in the pseudotrajectory. The flag -m2 gives the name of the file with the other molecule, which will be mobile in the simulation. Finally, the user needs to specify the rotational grid in form -rotgrid algorithm_N (see algorithm names above) and the translational grid after the flag -transgrid in one of the following formats: a list of distances (in nm), linspace(start, stop, num) or range(start, stop, step). The argument should be surrounded by quotation marks. Some acceptable translation grid arguments would be:

-transgrid "(1, 3, 5)" -> use distances 1nm, 3nm and 5nm

-transgrid "linspace(1, 3, 5)" -> use 5 equally spaced points between 1nm and 3nm

-transgrid "range(1, 3, 0.5)" -> use distances between 1nm and 3nm in 0.5nm increments

All flags starting with -- are optional and can be omitted for faster calculations. Remember that you can always add the flag --help to get further instructions.****

Using outputs

The pseudotrajectory .gro files can be used as regularly generated .gro files. We show how they can be displayed with VMD or used for GROMACS calculations, but the user is free to use them as inputs to any other tool.

Displaying pseudotrajectory

To display the example pseudotrajectory we created in the previous section with VMD, stay in the same directory and run

vmd output/pt_files/H2O_NH3_cube3D_15_full.gro

or on a windows computer

start vmd output/pt_files/H2O_NH3_cube3D_15_full.gro

Then, to fully display a pseudotrajectory, it is often helpful to change the display style and to display several or all frames at once. We suggest using the following commands within the VMD command line:

mol modstyle 0 0 VDW
mol drawframes 0 0 1:1:241

The first one displays the molecules as spheres with van der Waals radii and the second draws frames of the pseudotrajectory in a form <start>:<step>:<stop>. A useful trick is to use the number of rotations as <step> (in this case that would be 15) - this displays one structure per mass point without considering internal rotations. This number is also written in the name of the .gro file. If you want to display all frames, you can use any large number for <num_frames>, it does not need to correspond exactly to the number of frames.

Calculating energy along a pseudotrajectory

Often, a pseudotrajectory is used to explore where reagions of high and low energies lie when molecules approach each other. Since a range of timesteps sampling important rotations and translations is already provided in a PT, there is no need to run a real simulation. Therefore, the flag -rerun is always used while dealing with PTs in GROMACS. This setting saves time that would else be used for running an integrator and propagating positions.

To use GROMACS with PTs, the user must also provide a topology file which includes both molecules used in a pseudotrajectory. We will assume that this file is named topol.top. Lastly, we need a GROMACS run file that we will name mdrun.mdp. This file records GROMACS parameters and can be used as in normal simulations, but note that some parameters (e.g. integrator, dt) are meaningless for a pseudotrajectory without actual dynamics. Then, the energy along a pseudotrajectory can be calculated as follows:

gmx22 grompp -f mdrun.mdp -c H2O_NH3_cube3D_15_full.gro -p topol.top -o result.tpr   
gmx22 trjconv -f H2O_NH3_cube3D_15_full.gro -s result.tpr -o result.trr
gmx22 mdrun -s result.tpr -rerun result.trr
gmx22 energy -f ener.edr -o full_energy.xvg

Complex applications: using python package

Users who would like to build custom grids, pseudotrajectories or sets of rotations can import molgri as a python package (following installation described above) and work with all provided modules. Documentation is provided in form of docstrings or available in a compiled version at our github repository in docs folder.