This keyword is used to initialize the velocities of the atoms in the system according to a given temperature.


  • This keyword can be used in the following ways:

    velocity <initial_temperature>
    velocity <initial_temperature> seed <seed_number>
  • The temperature is in units of kelvin (K).

  • You can specify a seed for the random number generator to generate a fixed temperature distribution, which is optional. If not specified, the velocities are initialized differently each time.


The command:

velocity 10

means that one wants to set the initial temperature to 10 K.


  • The initial velocities generated in this fashion do not obey the Maxwell distribution. This is, however, not a problem since during the MD simulation, the Maxwell distribution will be achieved automatically within a short time.

  • The total linear and angular momenta are set to zero.

  • If there are already velocity data in the simulation model file, the initial temperature will not be used and the velocities will be initialized as those in the simulation model file. Important: In this case, you still need to write this keyword, otherwise the velocities will not be initialized at all (and will hence be undefined).