dynamics dyn_style energy_min_freq damping_coefficient
dyn_style= ld or qd or vv
ld is Langevin dynamics qd is quenched dynamics vv is Velocity Verlet
energy_min_freq= positive integer
damping_coefficient= positive real number
dynamics ld 300 1. dynamics qd 500 5.
This command sets the style of the dynamic run in CAC simulations.
dyn_style = ld, the Langevin dynamics is performed, i.e.,
where is the normalized lumped mass or the atomic mass, is the nodal/atomic position, is the equivalent nodal/atomic force, is the
damping_coefficient in ps, and is the time in ps. The Velocity Verlet form is employed to solve the equations of motion, as given in Eqs. 1-3 in Xu et al., 2016. The velocity is updated in
The ld style is used to keep a constant temperature in CAC simulations by adding to the force a time-dependent Gaussian random variable with zero mean and variance of , where is the atomic mass, is the Boltzmann constant (), is the temperature in K, and is the
time_step in ps. The random variable is calculated and added to the force in
langevin_force.f90. Note that when , the equation above reduces to
which is the equation of motion in damped molecular dynamics.
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.
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.
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.
hybrid.f90, among many
dynamics vv 500 1.