dynamics
Syntax
dynamics dyn_style energy_min_freq damping_coefficient

dyn_style
= ld or qd or vvld is Langevin dynamics qd is quenched dynamics vv is Velocity Verlet

energy_min_freq
= positive integer 
damping_coefficient
= positive real number
Examples
dynamics ld 300 1.
dynamics qd 500 5.
Description
This command sets the style of the dynamic run in CAC simulations.
When dyn_style
= ld, the Langevin dynamics is performed, i.e.,
where m is the normalized lumped mass or the atomic mass, \mathbf{R} is the nodal/atomic position, \mathbf{F} is the equivalent nodal/atomic force, \gamma is the damping_coefficient
in ps^{1}, and t is the time in ps. The Velocity Verlet form is employed to solve the equations of motion, as given in Eqs. 13 in Xu et al., 2016. The velocity \dot{\mathbf{R}} is updated in langevin_vel.f90
.
The ld style is used to keep a constant temperature in CAC simulations by adding to the force \mathbf{F} a timedependent Gaussian random variable \Theta(t) with zero mean and variance of \sqrt{2m\gamma k_\mathrm{B} T/\Delta t}, where m is the atomic mass, k_\mathrm{B} is the Boltzmann constant (8.6173324\times 10^{5} \mathrm{eV/K}), T is the temperature in K, and \Delta t is the time_step
in ps. The random variable is calculated and added to the force in langevin_force.f90
. Note that when T = 0, the equation above reduces to
which is the equation of motion in damped molecular dynamics.
When dyn_style
= qd, the quenched dynamics is performed, in which
 if the force and velocity point in opposite directions, the velocity is zeroed, i.e.,
 otherwise, the velocity is projected along the direction of the force, such that only the component of velocity parallel to the force vector is used, i.e.,
Note that with the qd style, which was first used in Xu et al., 2016, the temperature is considered 0 K or very nearly so.
When dyn_style
= vv, a dynamic simulation following
is performed using the Velocity Verlet scheme.
Note that the vv style cannot be used to keep a constant temperature and the qd style cannot be used to keep a finite temperature. When boolean
= t, if the vv style is chosen and if, for a finite temperature, the qd style is chosen, the user will get a warning message.
The energy_min_freq
is the frequency with which the energy minimization is performed during a dynamic run. This is relevant only if simulator_style
= hybrid.
Related commands
Related files
dynamics_init.f90
, dynamics.f90
, langevin_dynamics.f90
, quenched_dynamics.f90
, hybrid.f90
, among many
Default
dynamics vv 500 1.