GROMACS Wizard Light


In this tutorial I will present you GROMACS Wizard Light, the light version of the GROMACS Wizard Element. As for the full version, this element allows you to prepare and simulate systems using the well-known GROMACS package. With this Element, the main GROMACS features are now integrated directly in SAMSON. For example, it will help you to easily run GROMACS simulations and get results as plots and simulation trajectories.

Moreover, you will not have to compile/install the GROMACS package itself as this module comes with the latest version (5.1) of this package already installed. In addition, this module along with its pre-installed GROMACS package is available on Windows, Linux and Mac.

Installing GROMACS Wizard Light

First, make sure you have the GROMACS Wizard Light SAMSON Element that you can find in SAMSON Elements. Once you’re signed in, you should be able to add a SAMSON Element in one click (see User guide: installing SAMSON Elements if you need help with installing a new SAMSON Element). If you need help with installing SAMSON itself, please visit User guide: installing SAMSON.

Note: you can check whether you have this SAMSON Element in at least three different ways:

  1. In the Apps menu of SAMSON
  2. In the Edit / Preferences / Installation / Updates list in SAMSON
  3. Under My SAMSON / My Elements when you are signed in at SAMSON Connect

Let’s start the presentation of the many functionalities that this SAMSON element offers you.

For information, the PDB code of the structure used in this tutorial is 1AKI.



In order to launch GROMACS simulations, you need first to prepare your system, i.e. create a GROMACS model and define a simulation box. Then, you have the possibility to add explicit water molecules and neutralize the system charge by adding positive or negative ions. This is optional since you can build a system which contains implicit water molecules. Also, it is possible to include several structures in your GROMACS system. To do so, these structures need to be imported in your SAMSON document. Then, you can either select the ones you want to include in your system or leave the selection empty. If the selection is left empty while several structures are present in the document, SAMSON will ask you to choose if you want or not to include all the structures in your GROMACS system.

Generate model

This is the first step in the preparation of your GROMACS system. In the Model group box, you can select your force field from a list of available force fields. Each force field proposes water models which can be used to describe water molecules if you want to simulate your structure in explicit water. Also, each force field has positive and negative ions for the neutralization of your system. Concerning hydrogen atoms, we recommend you to ignore the ones already present in your structure, GROMACS will then add them using the selected force field naming convention (because not all the present force fields have the same H atoms naming convention).

The latest and most popular force fields (amber, charmm, gromos and oplsaa) can be found in this standard GROMACS version. In the GROMACS Wizard version you can also add custom GROMACS force fields, for more information about this feature, please visit the GROMACS Wizard tutorial

Define box

GROMACS automatically centers your structure inside the box and computes the box dimensions based on structure-box edge minimum distance. This distance represents the distance between the “outer” atom of the structure in each axis and the box edge. In SAMSON, you have the possibility to refine the box dimensions of your system before launching the preparation based on the distance between the structure and the box. The length of the box can then be modified to increase or decrease the box size. To help you define the box dimensions, the structure diameter (the largest distance between atoms) is displayed. You can also prepare your system without modifying your box size beforehand. In this case, the box dimensions will be set automatically based on the distance between the structure and the box.

Note that, when using periodic boundary conditions, a box edge distance of 1.0 nm minimum is recommended to avoid the structure to see its periodic image during the simulation. Specifying a solute-box distance of 1.0 nm will mean that there is at least 2.0 nm between any two periodic images of a protein. This distance will be sufficient for just about any cutoff scheme commonly used in simulations. For now, only cubic boxes can be defined in SAMSON but other GROMACS box shapes (like the rhombic dodecahedron) will be added in the future version of the element.

Add solvent and ions

In the Prepare tabulation, you have the possibility to add solvent to your system by checking the Add solvent checkbox to enable adding water molecules. Note that the model of water molecules that will be used (e.g. tip3p) is specified in the Model group box. If you choose “none” in the water model list, adding solvent and ions functionalities will be disabled.

We now need to make sure the system is neutral, by adding positive or negative ions. To run the neutralization step, you need to tick the Neutralize system group box and choose the positive and negative ions which will be used by GROMACS to reach a neutral total charge of your system. Each force field has a set of ions which can be used for the neutralization. Note that only one type of ions (positive or negative) or none of them will be added by GROMACS based on the total charge of your system which can be positive, negative or neutral. If you want to know the total charge of your system before running the preparation, you can compute it by pressing the update button . For example, if your system’s total charge is 8, GROMACS will add 8 negative ions to neutralize the system.

You can also ask GROMACS to increase the ions concentration in your system by adding more ions. If you check the Add additional ions group box you can specify the number of additional positive/negative ions to add. For example, if your system’s total charge is 8, you can ask GROMACS to add 12 additional positive ions thus bearing the total charge of the system to 20. Since GROMACS needs to neutralize the system it will add 20 negative ions during preparation. Therefore, the ions which would be added in your system to neutralize its total charge goes from 8 negative ions to 20 negative ions and 12 positive ions. Finally, note that all these ions will be added to your system as a replacement of water molecules.

Preparation result

After setting your model, box, solvent and ions preferences you can run the preparation by pressing on the Prepare button. A few seconds later a new structural model presenting the prepared GROMACS system will be created and imported in the SAMSON viewport. Note that you can choose to remove the initial structure by checking the Remove initial structure checkbox before pressing on the Prepare button. In this example, you can see that GROMACS added 20 negative ions and 12 positive ions to neutralize the system as we asked. In case you didn’t define your simulation box before pressing on Prepare, like in this example, GROMACS set the box dimensions using the provided distance to box edge.

Run simulations

The GROMACS Wizard Light Element also allows you to easily launch GROMACS minimization without having to learn the GROMACS syntax and procedures. In this section, I will show you how to perform energy minimization of your system. When launching minimization the model and box generations are performed again before starting these simulations. This workflow allows you to always start from the structure which is displayed in the SAMSON viewport. This way, you can freely modify your system in SAMSON between two steps (e.g. after preparation and before energy minimization). Note that launching GROMACS equilibration and simulation are possible using GROMACS Wizard element. To know more about these features please visit the GROMACS Wizard tutorial.


Once you have prepared your system, energy minimization can be performed in one click to remove steric clashes or incorrect geometries from your system. Most of the GROMACS molecular dynamics parameters are presented in Parameters section of the Minimize tabulation. By default, these parameters are populated with standard values that are suitable for many energy minimization simulations. Obviously, you can modify these parameters as you wish. In the Parameters group box, you will find the parameters that are likely to be changed often, like the energy minimization tolerance. The other GROMACS energy minimization parameters can be accessed by clicking on the button All… To know more about these parameters, please check the “apply custom parameters” section of this tutorial.

Thanks to SAMSON, you can import the GROMACS simulation trajectories like the minimization. For that you need to check the Load trajectory checkbox. To learn more about the simulation trajectories of GROMACS and the post-treatment which can be applied to them, please check the “trajectory” section of this tutorial. If you do not want to import the energy minimization trajectory of your system, uncheck the Load trajectory checkbox. In that case, the final step of your minimization will be imported instead of the whole trajectory. In the animation below, the energy minimization computation time (less than 3 min for 33 828 atoms) was skipped. Please note that during this calculation you can still use SAMSON thanks to the job manager of the GROMACS Wizard Light element. To know more about this job manager, please go the “Manage jobs” section of this tutorial.

Also, once energy minimization has converged, a plot describing the evolution of the system’s potential energy at each minimization step is imported and displayed in the Plots section. In this example, this plot demonstrates the nice, steady convergence of the potential energy. To know all the plotting functionalities of this SAMSON Element please refer to the “plots” section of this tutorial. 

Manage simulation results


GROMACS energy minimization (as well as equilibration and production simulation in GROMACS Wizard element) generate plots that helps you analyse these simulations. To be able to read and modify these plots you need to have Gnuplot installed on your system. Gnuplot is completely free and available on the three platforms: Windows, Linux and Mac. You can check the “Manage settings and output” section of this tutorial to learn how to set the Gnuplot version that will be used by this SAMSON element. 

When your GROMACS simulation in SAMSON is finished the corresponding plot is automatically added in a plots list that you can find in the Plots section of the energy minimization. The evolution of the potential energy of your system is then plotted to help you decide if you need to minimize your system again or not. You can interact with these plots in two different ways. First, you can right-click on the plot to display the possible actions that you can apply. You can open, save (in svg and in png formats), open the plot settings or remove this plot from the plots list. Second, you can double click on the plot (or right-click and choose Open) to open a window containing a large format of the targeted plot as well as the actions found in the right-click context menu and buttons to navigate through the plots present in the plot list.

If you open the plot setting you can change several aspects of the plot like the plot type or the plot color.


If you enabled periodic boundary conditions (PBC) during your simulation, the resulted trajectory might show non-realistic residue bonds for residues located near the box edges which can therefor move to the other side of the box. Below is an energy minimization trajectory imported with no post-treatment (none).

Using GROMACS it is possible to treat these trajectories with one of the post-treatments listed in the PBC treatment type dropdown list. This post-treatment allows you to generate more realistic trajectories.

Advanced features

Apply custom parameters

The most used GROMACS energy minimization parameters have been directly populated in the Parameters section of the minimize tabulation. However, you can access and modify all the GROMACS energy minimization parameters by clicking on the button (All.. button).

A new window will be opened with a list of parameters groups, you can click on each group to display the parameters which they contain. Special care has been made to describe all the parameters in the most understandable way possible. Almost all the energy minimization parameters are present in the advanced parameters window. In the GROMACS Wizard Element, you can add and manage parameters that are not presented. To know more about these features please visit the GROMACS Wizard tutorial. To apply your modifications of the parameters you have to click on the button Ok while the Cancel button discards all the modifications that you have done in the advanced parameters window except the resetting to the default parameters. Finally, you can reset the parameters to default values at any time by clicking on the Reset button.

In addition to the comments provided, more explanations of the parameters used can be found in the GROMACS simulation parameters documentation.

Manage jobs

The GROMACS Wizard Light element allows you to run an energy minimization but still use GROMACS to perform other tasks on SAMSON. In the GROMACS Wizard Light version, the job manager allows you to stop an energy minimization job. Jobs display some information like the name, status (running, queued, finished, canceled or stopped) and other useful information. In the GROMACS Wizard version you can do more. You can, for example, run several simulations types (e.g. in explicit or implicit water) one after the other thanks to its queuing system. You can check these features in the GROMACS Wizard tutorial

Many actions can be applied to these jobs. You can rename the job by double-clicking on the job’s name cell. You can simply stop it (or totally remove it from the list) by right-clicking on the job and pressing on Stop (or Remove if you want to remove it).

Also, before the job finishes, you can change its trajectory importation options. Finally, you can stop all the jobs using the Stop all button or you can stop and clear using the Clear button.

Manage settings and output

You can display the settings of this element by pressing on the button . There you can see that the shipped GROMACS package is a version 5.1. Using GROMACS Wizard, you can use another version of GROMACS by providing the path to the executable gmx.exe (or gmx for Linux and Mac). Check out this feature in the GROMACS Wizard tutorial.

Also, you can provide the path to the Gnuplot program if you want to display and manage GROMACS plots.

During preparation or simulation of your GROMACS systems, the GROMACS package output some useful informations. You can find this output in the GROMACS output window by pressing on the button . There you will be able to save the GROMACS output in a .txt file by pressing on the Save button or to clear it by pressing on the Clear button.

To add this SAMSON Element to you SAMSON follow the GROMACS Wizard Light link or check it out on the User guide: installing SAMSON.

If you need help with installation of SAMSON Element, please visit the User guide: installing SAMSON Elements.

For remarks or suggestions, please let us know in the comments below or on the SAMSON forum.



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