Hi friends... @dexterdev is back with his Classical Molecular Dynamics Series. In case you missed my last article in this series, please see it here:
Classical Molecular Dynamics Series [Part - 4a]: Let us set up a simulation and run it!
Today we will simulate a small system which anyone can try out on their computer, provided you have installed NAMD and VMD. We will start from the psf and pdb file which we created in the last article using CHARMM-gui. I am linking those files here: step5_assembly.psf, step5_assembly.pdb
Let us keep things simple this time since we are simulating the system in laptop/desktop etc. Mine is an i7 processor, 8GB RAM machine. We will extract a single lipid from the above psf and pdb, add water and ions to it and then simulate it under 2 conditions.
- Condition 1: Temperature-273K
- Condition 2: Temperature-473K
(There are certain caveats here. High-temperature simulations are here to demonstrate the higher agitation in the molecules.)
Extracting a single lipid
Load VMD with $vmd step5_assembly.psf step5_assembly.pdb
Then load TCl-Console and type as below
set sel [atomselect 0 "resname DMPC and resid 1"]
$sel writepdb lipid.pdb
$sel writepsf lipid.psf
The above lines of code will pull out a lipid with resid 1 and write to a new psf and pdb file. For those who want to know more about DMPC phospholipid, please see my article on it here.
Adding water and ions to the system
Load the lipid.psf and lipid.pdb into VMD.
Go to Extensions -> Modeling -> Add Solvation Box.
Now give a suitable output name. I gave the name as lipid_solvate.
Now fill the all 6 min and max entries of Box padding with 10 (Angstroms is the unit.):
This will prevent the lipid's' interaction with its own image. Ooh... Now, what does that mean? Short answer: So we have simulations under periodic boundary conditions. Which means each box of the simulation system is repeated in the 3 dimensions. So the total charge in the system must be zero, else the charge won't converge.
Once you press the Solvate button, a water box will be generated on top of lipid. Now we have to add ions to neutralize the system. This is pretty straightforward. Go to Extensions -> Modeling -> Add Ions. I chose NaCl with 0.15 mol/liter. Choose an out output name as in the solvation case. The name I selected is dmpc_solvate_ionized. So you have psf and pdb files corresponding to lipid in water and ions system.
The resultant system will look like below:
Simulation part
In the config file, you need to specify the periodic box size. To compute it you can use the Tcl script below:
get_cell.tl
proc get_cell {{molid top}} {
set all [atomselect $molid all]
set minmax [measure minmax $all]
set vec [vecsub [lindex $minmax 1] [lindex $minmax 0]]
puts "cellBasisVector1 [lindex $vec 0] 0 0"
puts "cellBasisVector2 0 [lindex $vec 1] 0"
puts "cellBasisVector3 0 0 [lindex $vec 2]"
set center [measure center $all]
puts "cellOrigin $center"
$all delete
}
Load the above tcl script in Tcl-console after loading the psf and pdb file, by
source ./get_cell.tcl
get_cell
Let us start with config file for the 273K temperature case:
##############################################################
## JOB DESCRIPTION ##
#############################################################
# 1 DMPC molecule in water and ions
#############################################################
## ADJUSTABLE PARAMETERS ##
#############################################################
structure ./dmpc_solvate_ionized.psf
coordinates ./dmpc_solvate_ionized.pdb
set temperature 273
set outputname DMPC
firsttimestep 0
#############################################################
## SIMULATION PARAMETERS ##
#############################################################
# Input
paraTypeCharmm on
parameters ./par_water_ions.prm
parameters ./par_all36_lipid.prm
temperature $temperature
# Force-Field Parameters
exclude scaled1-4
1-4scaling 1.0
cutoff 12.
switching on
switchdist 10.
pairlistdist 15.0
margin 8.0
# Integrator Parameters
# fullElectFrequency*timestep <=4.0
# stable time steps:
# with rigidBonds=all: electro:6fs;short-rangeNB:2fs;bonded:2fs
# otherwise : 2fs 2fs 1fs
timestep 2.0 ;# 2fs/step
rigidBonds all ;# Needed for 2fs steps
useSettle on
nonbondedFreq 1
fullElectFrequency 2
stepspercycle 10
#########################
zeroMomentum no
#########################
# Constant Temperature Control
langevin off ;# do langevin dynamics
langevinDamping 5 ;# damping coefficient (gamma) of 5/ps
langevinTemp $temperature
langevinHydrogen off ;# don't couple langevin bath to hydrogens
#temperature coupling for temperature coupling
tCouple on
tCoupleTemp $temperature
if {1} {
# Periodic Boundary Conditions
# center of the periodic box; NOT ORIGIN!!!
# paste the output of get_cell.tcl here:
cellBasisVector1 29 0 0
cellBasisVector2 0 30 0
cellBasisVector3 0 0 41.5
cellOrigin -3.1667470932006836 -3.8784143924713135 10.219316482543945
}
wrapAll on
# PME (for full-system periodic electrostatics)
# multiples of 2,3,5 & >=dimensions above
PME yes
PMEGridSizeX 30
PMEGridSizeY 30
PMEGridSizeZ 60
# Constant Pressure Control (variable volume)
useGroupPressure yes ;# needed for rigidBonds
useFlexibleCell no
useConstantArea no
langevinPiston on
langevinPistonTarget 1.01325 ;# in bar -> 1 atm
langevinPistonPeriod 100.
langevinPistonDecay 50.
langevinPistonTemp $temperature
# Output
outputName $outputname
restartfreq 500000 ;# every 1 ns
dcdfreq 10000 ;#
xstFreq 10000
outputEnergies 10000
outputPressure 10000
#############################################################
## EXECUTION SCRIPT ##
#############################################################
# Minimization
minimize 5000
reinitvels $temperature
run 1000000 ;# for 2 ns
Assuming parameter files are in the current folder, open the terminal and type:
namd2 DMPC_NPT_eq.config > out. It took around 6 hours to finish the simulation(3109 atoms, 2ns simulation). After the simulation, you can load psf and dcd output to get the trajectory. Now you can repeat the entire simulation with a higher temperature by just replacing 273 to a higher number. The gif file in the top of the article shows two systems simulated as described.
It is quite obvious that the heated system fluctuates more vigorously. But we need to quantify it, right? In the next part, we will see more analysis related stuff.
Files
All input files and output files are uploaded in my github page for refreence. LINK TO FILES
References [Books for further reading]
Textbook references for learning theory of Molecular Dynamics:
- "Statistical Mechanics: Theory and Molecular Simulations" by Mark E. Tuckerman
- "Molecular Modelling: Principles and Applications" by Andrew R. Leach
- "Computer Simulation of Liquids" by D. J. Tildesley and M.P. Allen
References specific to NAMD and VMD:
CHARMM-gui related papers:
Join #steemSTEM
#steemSTEM is a community project with the goal to promote and support Science, Technology, Engineering and Mathematics related content and activities on the STEEM blockchain. If you wish to support the #steemSTEM project you can: Contribute STEM content using the #steemstem tag | Support steemstem authors | Join our curation trail | Join our Discord community | Delegate SP to steemstem
Convenient Delegation Links:
Follow me @dexterdev
____ _______ ______ _________ ____ ______
/ _ / __\ \//__ __/ __/ __/ _ / __/ \ |\
| | \| \ \ / / \ | \ | \/| | \| \ | | //
| |_/| /_ / \ | | | /_| | |_/| /_| \//
\____\____/__/\\ \_/ \____\_/\_\____\____\__/
By the way, this is a nice logo which @mathowl created for me. It is well suited for my MD series. :). Thank you @mathowl.
