.
Thus we have:
Here Pd is the desired pressure and tp is the time constant for pressure fluctuations and 
. At each step the volume of the cell is scaled by a factor of
and the molecular centre of mass by a factor 
.Thus we have:
This method when combined with a method of temperature control creates trajectories in the NPT ensemble. For the NPT ensemble Hoover (1985) gave the following equations for pressure control.
here 
is given by Eq 49b. is the Boltzmann constant and f is the frictional force. Here we have to specify the desired pressure Pd, time constant tp, decay time 
and instantaneous temperature 
of the piston. In addition, the damping coefficients and desired temperature of the molecule T are also specified for maintaining the constant temperature. For bulk solvent/fluid the instantaneous pressure of a simulation cell will have mean square fluctuations of kT/ (Vβ) where β is the isothermal compressibility, having root mean square (RMS) of 100 bar for 10,000 atom bimolecular system [Lifshitz and Landau,1985].