Nonadiabatic processes

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nonadiabatic_processes sets the option to simulate the non-equilibrium phenomena, for example under a radiation cascade, where the electronic stopping and electronic-phonon coupling in the later stages of the event will be taken into account according to the eph model [[1]]. MD for the atoms is performed through the use of generalized Langevin dynamics where the random and friction forces are related through the fluctuation-dissipation theorem. Here the random and friction forces are given as tensor functions of atomic positions. For the atoms, the force equation that is solved is given as follows:

[math]\displaystyle{ \mathbf{F}_I {{=}} -\mathbf{\nabla}_IU_{adiab} - \sum_J B_{IJ}\mathbf{v}_J + \sum_J W_{IJ}\mathbf{\xi}_J }[/math]

The electronic system is considered as a mesh in three dimension and electronic temperature is calculated along with MD by using the finite difference method to solve the non-linear heat diffusion equation:

[math]\displaystyle{ C_e(T_e)\frac{\partial T_e}{\partial t} = \mathbf{\nabla}.(\kappa_e(T_e)\mathbf{\nabla}T_e) + Q_{ei} }[/math]

Here, [math]\displaystyle{ Q_{ei} }[/math] is a local energy transfer term between the atoms and electrons, which may or may not be combined with an external source term. See more on this and temperature-dependent electronic parameters in the related keyword eph_Tinfile. There are twenty input keywords connected to this calculation method. For some of these keywords input may or may not be provided always. These are as follows:

eph_fdm_option, eph_friction_option, eph_random_option, eph_betafile, eph_Tinfile, eph_box_limits, eph_rho_e, eph_C_e, eph_kappa_e, eph_Ti_e, eph_gsx, eph_gsy, eph_gsz, eph_fdm_steps, eph_md_last_step, eph_md_prev_time, eph_E_prev_time, eph_freq_Tout, eph_freq_mesh_Tout, eph_Toutfile


Summary for nonadiabatic_processes keyword
Required/optional Type Accepted values Default See also Remarks
Optional Boolean .true. or .false. .false. electronic_stopping Required functionality for doing non-equilibrium MD simulations


nonadiabatic_processes = .true.
eph_fdm_option = 1
eph_friction_option = 1
eph_random_option = 1
eph_betafile = 'CouplingFile.beta'
eph_Tinfile = 'T_input.fdm'
box_limits = -30.0 30.0 -30.0 30.0 -30.0 30.0    ! in Ang
eph_rho_e = 1.0
eph_C_e = 3.5E-01    ! in eV/K/Ang^3
eph_kappa_e = 0.12   ! in eV/K/Ang/ps
eph_Ti_e = 50.0      ! in K
eph_gsx = 4
eph_gsy = 4
eph_gsz = 4
eph_fdm_steps = 1
eph_md_last_step = 0
eph_md_prev_time = 0.0    ! in fs
eph_E_prev_time = 0.0     ! in eV
eph_freq_Tout = 10
eph_freq_mesh_Tout = 1000
eph_Toutfile = 'T-mesh.out'