How to start a simulation

In Monte Carlo simulations, an initial condition must be generated at the start of the simulation, defining positions for all particles. In molecular dynamics simulations, besides the positions, the iiutial velocities must be defined. 

The necessary amount of care for generating the initial condition (r(0), y 0)) of a simulated system depends on the relevant time scales of interesting motions. For a simple fluid of Lennard-Jones particles at room temperature, the particles move fast enough for the system to quickly reach equilibrium regardless of the initial configuration. Consequently, the simplest-to-build configuration may be chosen. This could be, for example, a simple FCC lattice with the particles originally placed 

In molecular dynamics, the initial condition must include the velocities. These can be drawn from a Maxwell-Boltzmann distribution. 

Care must be taken so that the total momentum of the system in aU directions is zero. This can be achieved by choosing the velocity of the last particle, so that 

On the other hand, for complex systems it is imperative to generate an initial condition that is as close to the equilibrium structure as possible. For example, if a mixture of water and alcohol molecules are simulated at room temperature, the molecules may be best randomly placed at the beginning of the simulation. Since the two liquids mix well, a random initial configuration may be preferable to a lattice one with the molecular phases separated. 

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