# Dynamics in the ground state¶

[Input: recipes/moleculardynamics/md/]

Dynamics on the Born-Oppenheimer ground state energy surface can be performed in DFTB+ by setting the input geometry driver to be VelocityVerlet

Driver = VelocityVerlet{
TimeStep [fs] = 1.0
Thermostat = NoseHoover {
Temperature [Kelvin] = 400
CouplingStrength [cm^-1] = 3200
}
Steps = 20000
MovedAtoms = 1:-1
MDRestartFrequency = 100
}


The velocity Verlet driver should have a time step on the scale of ~10x the highest vibrational period in the system. 1 fs is a common choice. The

The input file specifies initial velocities of the atoms

Velocities [AA/ps] {
0.63060001     10.71652407      0.41599521
-4.78167517     -0.67726160      6.81193886
.
.
}


## Thermalising a system¶

[Input: recipes/moleculardynamics/thermalise/]

The initial velocities of atoms can be user supplied, however it is more common to generate them by thermalising the system starting from an initial Maxwell-Boltzmann distribution of atomic velocities. These can be generated for example by using the following input:

Thermostat = NoseHoover {
# Target temperature
Temperature [Kelvin] = 400
# Approximately the highest vibrational frequency of the molecule
CouplingStrength [cm^-1] = 3200
}


within the VelocityVerlet input block. The initial values of the velocities are set from a random number generator, hence to make the calculation repeatable this is set to a specific value

Options = {
RandomSeed = 3871906
}


However, for real calculations, it would be common to use a fully random choice, by omitting the RandomSeed value.