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The aim of this tutorial is to introduce you to the PLUMED syntax. We will go through the writing of input files to calculate and print simple collective variables on a pre-calculated trajectory.
Once this tutorial is completed, students will be able to:
You can find detailed instructions about how to install PLUMED here:
https://github.com/plumed/conda
Let's compile it together!
PLUMED is a library that can be incorporated into many MD codes by adding a relatively simple and well documented interface. Once it is incorporated you can use PLUMED to perform a variety of different analyzes on the fly and to bias the sampling in the molecular dynamics engine. PLUMED can also, however, be used as a standalone code for analyzing trajectories. If you are using the code in this way you can, once PLUMED is compiled, run the PLUMED executable by issuing the command:
plumed <instructions>
Let's start by getting a feel for the range of calculations that we can use PLUMED to perform. Issue the following command now:
plumed --help
What is output when this command is run is a list of the tasks you can use PLUMED to perform. There are commands that allow you to patch an MD code, commands that allow you to run molecular dynamics and commands that allow you to build the manual. In this tutorial we will mostly be using PLUMED to analyze trajectories, however. As such most of the calculations we will perform will be performed using the driver tool. Let's look at the options we can issue to PLUMED driver by issuing the following command:
plumed driver --help
As you can see we can do a number of things with PLUMED driver. For all of these options, however, we are going to need to write a PLUMED input file. The syntax of the PLUMED input file is the same that we will use later to run enhanced sampling simulations, so all the things that you will learn now will be useful later when you will run PLUMED coupled to an MD code.
The main goal of PLUMED is to compute collective variables, which are complex descriptors than can be used to analyze a conformational change or a chemical reaction. This can be done either on-the-fly during molecular dynamics or a posteriori, using PLUMED as a post-processing tool. In both cases one, should create an input file with a specific PLUMED syntax. A sample input file is below:
# Compute distance between atoms 1 and 10. # Atoms are ordered as in the trajectory files and their numbering starts from 1. # The distance is called "d" for future reference. d: DISTANCE ATOMS=1,10 # Create a virtual atom in the center between atoms 20 and 30. # The virtual atom only exists within PLUMED and is called "center" for future reference. center: CENTER ATOMS=20,30 # Compute the torsional angle between atoms 1, 10, 20, and center. # Notice that virtual atoms can be used as real atoms here. # The angle is called "phi" for future reference. phi1: TORSION ATOMS=1,10,20,center # the same CV defined before can be split into multiple line TORSION ... LABEL=phi2 ATOMS=1,10,20,center ... # Print d every 10 step on a file named "COLVAR1". PRINT ARG=d STRIDE=10 FILE=COLVAR1 # Print phi1 and phi2 on another file names "COLVAR2" every 100 steps. PRINT ARG=phi1,phi2 STRIDE=100 FILE=COLVAR2
In the input file above, each line defines a so-called action. An action could either compute a distance, or the center between two or more atoms, or print some value on a file. Each action supports a number of keywords, whose value is specified. Action names are highlighted in green and, clicking on them, you can go to the corresponding page in the manual that contains a detailed description for each keyword. Actions that support the keyword STRIDE are those that determine how frequently things are to be done. Notice that the default value for STRIDE is always 1. In the example above, omitting STRIDE keywords the corresponding COLVAR files would have been written for every frame of the analyzed trajectory. All the other actions in the example above do not support the STRIDE keyword and are only calculated when requested. That is, d will be computed every 10 frames, and phi1 and phi2 every 100 frames. In short, you can think that for every snapshot in the trajectory that you are analyzing PLUMED is going to execute all the listed actions, though some of them are optimized out when STRIDE is different from 1.
Variables should be given a name (in the example above, d, phi1, and phi2), which is then used to refer to these variables. Lists of atoms should be provided as comma separated numbers, with no space. Virtual atoms can be created and assigned a name for later use.
You can find more information on the PLUMED syntax at Getting Started page of the manual. The complete documentation for all the supported collective variables can be found at the Collective Variables page.
By default the PLUMED inputs and outputs quantities in the following units:
If you want to change these units you can do this using the UNITS keyword.
Before starting the tutorial, please create a separate directory, named handson_1, and enter it:
mkdir handson_1 cd handson_1
The reference trajectories and other files needed for the exercises proposed in this tutorial can be downloaded from GitHub using the following command:
wget https://github.com/plumed/lugano2019/raw/master/handson_1/handson_1.tgz
The archive contains the following files:
The archive can be unpacked using the following command:
tar xvzf handson_1.tgz
This tutorial has been tested on PLUMED version 2.5.1.
In this exercise, we will make practice with computing and printing collective variables. To analyze the trajectories provided here, you should:
plumed.dat) similar to the one above.plumed driver command (see below)Notice that you can also visualize trajectories with VMD directly. For example, the trajectory traj-whole.xtc can be visualized with the command vmd GB1_native.pdb traj-whole.xtc.
Let's prepare a PLUMED input file that calculates
GB1_native.pdb file to retrieve the list of indexes of the CA atoms.R_0 to 8.0 A (be careful with units!!).Here you can find a sample plumed.dat file that you can use as a template. Whenever you see an highlighted FILL string, this is a string that you should replace.
# Compute gyration radius on CA atoms: r: GYRATION ATOMS=__FILL__ # Compute number of contacts between CA atoms co: COORDINATION GROUPA=__FILL__ R_0=__FILL__ # Print the collective variables on COLVAR file every step PRINT ARG=r,co FILE=COLVAR STRIDE=__FILL__
Once your plumed.dat file is complete, you can use it with the following command
> plumed driver --plumed plumed.dat --mf_xtc traj-broken.xtc
Scroll in your terminal to read the PLUMED log. As you can see, PLUMED gives a lot of feedback about the input that he is reading. There's the place where you can check if PLUMED understood correctly your input.
The command above will create a COLVAR file like this one:
#! FIELDS time r co 0.000000 2.458704 165.184127 1.000000 2.341932 164.546604 2.000000 2.404708 162.606975 3.000000 2.454297 143.850122 4.000000 2.569342 147.110408 5.000000 2.304027 163.608703
Notice that the first line informs you about the content of each column.
In case you obtain different numbers, check your input, you might have made some mistake!
This file can then be visualized using gnuplot
gnuplot> p "COLVAR" u 1:2, "" u 1:3
Now, look at what happens if you run the exercise twice. The second time, PLUMED will back up the previously produced file so as not to overwrite it. You can also concatenate your files by using the action RESTART at the beginning of your input file.
Finally, let's try to use the same input file with traj-whole.xtc and examine the results. Did you find the same results as with the previous trajectory? Why?
PLUMED provides some shortcuts to select atoms with specific properties. To use this feature, you should specify the MOLINFO action along with a reference pdb file.
Let's try to use this functionality to calculate the backbone dihedral angle phi of residue 2 of GB1. This CV is defined by the action TORSION and a set of 4 atoms. For residue i, the dihedral phi is defined by these atoms: C(i-1),N(i),CA(i),C(i). After consulting the manual and inspecting GB1_native.pdb, let's define the dihedral angle in question in two different ways: using the MOLINFO shortcut to define psi of residue 2 and with an explicit list of 4 atoms.
# activate MOLINFO functionalities MOLINFO STRUCTURE=__FILL__ # define phi dihedral of residue 2 as a list of 4 atoms t1: TORSION ATOMS=__FILL__ # define the same dihedral using MOLINFO shortcuts t2: TORSION ATOMS=__FILL__ # print the value of t1 and t2 every 10 steps PRINT ARG=__FILL__ FILE=COLVAR STRIDE=10
After completing the PLUMED input file above, let's use it to analyze the trajectory traj-whole.xtc that you downloaded at the start of this exercise. Now, let's plot the trajectory of the two CVs above. If you executed this exercise correctly, these two trajectories should be identical.
Sometimes, when calculating a CV, you may not want to use the positions of a number of atoms directly. Instead you may want to define a virtual atom whose position is generated based on the positions of a collection of other atoms. For example you might want to use the center of mass (COM) or the geometric center (CENTER) of a group of atoms.
In this exercise, you will learn how to specify virtual atoms and later use them to define a CV. Let's start by having a look at the PLUMED input file below.
# geometric center of first residue first: CENTER ATOMS=1,2,3,4,5,6,7,8 # geometric center of last residue last: CENTER ATOMS=427-436 # distance between centers of first and last residues, with PBCs d1: DISTANCE ATOMS=first,last # distance between centers of first and last residues, without PBCs d2: DISTANCE ATOMS=first,last NOPBC # print the two distances every step PRINT ARG=d1,d2 STRIDE=1 FILE=COLVAR
Make a PLUMED input containing the above input and execute it on the trajectory traj-broken.xtc by making use of the following command:
plumed driver --mf_xtc traj-broken.xtc --plumed plumed.dat
Before we turn to analyzing what is output from this calculation there are a few things to note about this input file. Firstly, I should describe what this file instructs PLUMED to do. It tells PLUMED to:
Notice that in the input above we have used two different ways of writing the atoms used in the CENTER calculation:
Notice also that ranges of atoms can be defined with a stride which can also be negative as shown by the commands below, which are both equivalent:
Let's now analyze the output of the calculation by plotting the time series of the two CVs. Are they identical? What do you think is happening in those frames in which the two CVs are different?
You can repeat the same analysis on traj-whole.xtc and compare with the previous trajectory. Are the results identical?
In summary, in this tutorial you should have learned how to use PLUMED to:
1.8.14