Langevin thermostat

The Langevin thermostat maintains the temperature through a modification of Newton's equations of motion

$\dot{r_i} = p_i/m_i \qquad \dot{p_i} = F_i - {\gamma}_i\,p_i + f_i,$

where F_i is the force acting on atom i due to the interaction potential, γ_i is a friction coefficient, and f_i is a random force simulating the random kicks by the damping of particles between each other due to friction. The random numbers are chosen from a Gaussian distribution with the following variance

$\sigma_i^2 = 2\,m_i\,{\gamma}_i\,k_B\,T/{\Delta}t$

with Δt being the time-step used in the MD to integrate the equations of motion. Obviously, Langevin dynamics is identical to the classical Hamiltonian in the limit of vanishing γ.

The friction coefficient is set by the LANGEVIN_GAMMA parameter.

The Langevin thermostat is selected by MDALGO=3.

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