=====Molecular dynamics=====

In this section we describe how to simulate molecular dynamics in NVT and NVE ensembles. In the canonical ensemble are N,V,T respectively N,V,E conserved. There are implemented two methods to hold temperature constant in fireball. The first one is velocity rescaling and the second is Nose- Hoover chain thermostat. These options are driven by keyword: ''iensemble'' that controls which statistical ensemble will used within the simulation. In particular, to run free dynamics without any structural optimization method, one needs to set ''iquench = 0''.

| iensemble=0 | NVE ensemble |
| iensemble=1 | NVT ensemble with velocity rescaling |
| iensemble=2 | NVT with a Nose-Hoover chain thermostat and velocity-verlet |
| iensemble=3 | NVE with velocity-verlet integrator |


Here is an example of answer.bas for benzene molecule:

==== answer.bas : ====
<code>
         12
   6     -4.183081     -4.008593     -0.000092
   6     -3.047805     -3.090020      0.004490
   6     -3.275844     -1.647830      0.001899
   6     -4.639523     -1.125588      0.002371
   6     -5.775305     -2.043508      0.000193
   6     -5.546619     -3.485699     -0.007522
   1     -6.432266     -4.204789     -0.015592
   1     -4.005535     -5.135519     -0.000976
   1     -1.983459     -3.500264      0.005183
   1     -2.389446     -0.929457     -0.002861
   1     -4.816925      0.001517     -0.000293
   1     -6.839935     -1.633156     -0.001403
</code>

In the case of Nose-Hoover thermostat we can choose number of chains and mass of extra additional degree of fredom. This values are set in file ''NH.optional'' placed in work directory.

==== NH.optional : ====
<code>
4               !number of chains
1.2             ! mass of chain 1
1.2             ! mass of chain 2
1.2             ! mass of chain 3
1.2             ! mass of chain 4
</code>

Temperature of system is set by keywords ''T_initial'' and ''T_final'' in section ''&OPTION&''. Time step of ionic motion is defined through a standard keyword ''dt'' in femtoseconds. An example of fireball.in input file for classical molecular dynamic is below.

==== fireball.in : ====
<code>
&OPTION
basisfile = answer.bas 
iquench = 0
iendtemp = 0
T_initial = 500
T_final = 500
iensemble = 2
icluster = 1
nstepf = 200
dt = 0.1
iqout = 3
ifixcharge = 0
&END

&OUTPUT
iwrtxyz = 1
&END
</code>
All input files here:{{:fireball:md:input_files.tar.gz|input_files}}
==== Output ====

The position and values of total energy and temperature are written into output file ''answer.xyz'' if the keyword ''iwrtxyz=1''. To simplier plotting the values of temperature we can import tepmerature value from the file ''answer.xyz'' to another file e.g. ''T_dyn'' by command:
<code>
awk '/ETOT/ {print 6$}' answer.xyz > T_dyn '' 
</code>
where are the data in simple list. 

Temperature during simulation of benzene hold on by velocity rescaling and Nose-Hoover thermostat during the simulation in Figure 1.