220 lines
8.3 KiB
ReStructuredText
220 lines
8.3 KiB
ReStructuredText
.. index:: fix electron/stopping
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.. index:: fix electron/stopping/fit
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fix electron/stopping command
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=============================
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fix electron/stopping/fit command
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=================================
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID style args
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* style = *electron/stopping* or *electron/stopping/fit*
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.. parsed-literal::
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*electron/stopping* args = Ecut file keyword value ...
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Ecut = minimum kinetic energy for electronic stopping (energy units)
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file = name of the file containing the electronic stopping power table
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*electron/stopping/fit* args = Ecut c1 c2 ...
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Ecut = minimum kinetic energy for electronic stopping (energy units)
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c1 c2 = linear and quadratic coefficients for the fitted quadratic polynomial
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* zero or more keyword/value pairs may be appended to args for style = *electron/stopping*
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.. parsed-literal::
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keyword = *region* or *minneigh*
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*region* value = region-ID
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region-ID = region whose atoms will be affected by this fix
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*minneigh* value = minneigh
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minneigh = minimum number of neighbors an atom to have stopping applied
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix el all electron/stopping 10.0 elstop-table.txt
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fix el all electron/stopping 10.0 elstop-table.txt minneigh 3
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fix el mygroup electron/stopping 1.0 elstop-table.txt region bulk
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fix 1 all electron/stopping/fit 4.63 3.3e-3 4.0e-8
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fix 1 all electron/stopping/fit 3.49 1.8e-3 9.0e-8 7.57 4.2e-3 5.0e-8
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Description
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"""""""""""
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This fix implements inelastic energy loss for fast projectiles in solids. It
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applies a friction force to fast moving atoms to slow them down due to
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:ref:`electronic stopping <elstopping>` (energy lost via electronic collisions per
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unit of distance). This fix should be used for simulation of irradiation
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damage or ion implantation, where the ions can lose noticeable amounts of
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energy from electron excitations. If the electronic stopping power is not
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considered, the simulated range of the ions can be severely overestimated
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(:ref:`Nordlund98 <Nordlund98>`, :ref:`Nordlund95 <Nordlund95>`).
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The electronic stopping is implemented by applying a friction force
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to each atom as:
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.. math::
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\vec{F}_i = \vec{F}^0_i - \frac{\vec{v}_i}{\|\vec{v}_i\|} \cdot S_e
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where :math:`\vec{F}_i` is the resulting total force on the atom.
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:math:`\vec{F}^0_i` is the original force applied to the atom, :math:`\vec{v}_i` is
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its velocity and :math:`S_e` is the stopping power of the ion.
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.. note::
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In addition to electronic stopping, atomic cascades and irradiation
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simulations require the use of an adaptive timestep (see
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:doc:`fix dt/reset <fix_dt_reset>`) and the repulsive ZBL potential (see
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:doc:`ZBL <pair_zbl>` potential) or similar. Without these settings the
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interaction between the ion and the target atoms will be faulty. It is also
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common to use in such simulations a thermostat (:doc:`fix_nvt <fix_nh>`) in
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the borders of the simulation cell.
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.. note::
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This fix removes energy from fast projectiles without depositing it as a
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heat to the simulation cell. Such implementation might lead to the unphysical
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results when the amount of energy deposited to the electronic system is large,
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e.g. simulations of Swift Heavy Ions (energy per nucleon of 100 keV/amu or
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higher) or multiple projectiles. You could compensate energy loss by coupling
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bulk atoms with some thermostat or control heat transfer between electronic and
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atomic subsystems with the two-temperature model (:doc:`fix_ttm <fix_ttm>`).
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At low velocities the electronic stopping is negligible. The electronic
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friction is not applied to atoms whose kinetic energy is smaller than *Ecut*\ ,
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or smaller than the lowest energy value given in the table in *file*\ .
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Electronic stopping should be applied only when a projectile reaches bulk
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material. This fix scans neighbor list and excludes atoms with fewer than
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*minneigh* neighbors (by default one). If the pair potential cutoff is large,
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minneigh should be increased, though not above the number of nearest neighbors
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in bulk material. An alternative is to disable the check for neighbors by
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setting *minneigh* to zero and using the *region* keyword. This is necessary
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when running simulations of cluster bombardment.
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If the *region* keyword is used, the atom must also be in the specified
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geometric :doc:`region <region>` in order to have electronic stopping applied to
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it. This is useful if the position of the bulk material is fixed. By default
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the electronic stopping is applied everywhere in the simulation cell.
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----------
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The energy ranges and stopping powers are read from the file *file*\ .
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Lines starting with *#* and empty lines are ignored. Otherwise each
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line must contain exactly **N+1** numbers, where **N** is the number of atom
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types in the simulation.
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The first column is the energy for which the stopping powers on that
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line apply. The energies must be sorted from the smallest to the largest.
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The other columns are the stopping powers :math:`S_e` for each atom type,
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in ascending order, in force :doc:`units <units>`. The stopping powers for
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intermediate energy values are calculated with linear interpolation between
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2 nearest points.
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For example:
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.. parsed-literal::
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# This is a comment
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# atom-1 atom-2
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# eV eV/Ang eV/Ang # units metal
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10 0 0
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250 60 80
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750 100 150
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If an atom which would have electronic stopping applied to it has a
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kinetic energy higher than the largest energy given in *file*\ , LAMMPS
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will exit with an error message.
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The stopping power depends on the energy of the ion and the target
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material. The electronic stopping table can be obtained from
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scientific publications, experimental databases or by using
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:ref:`SRIM <SRIM>` software. Other programs such as :ref:`CasP <CasP>` or
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:ref:`PASS <PASS>` can calculate the energy deposited as a function
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of the impact parameter of the ion; these results can be used
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to derive the stopping power.
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----------
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Style *electron/stopping/fit* calculates the electronic stopping power
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and cumulative energy lost to the electron gas via a quadratic functional
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and applies a drag force to the classical equations-of-motion for all
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atoms moving above some minimum cutoff velocity (i.e., kinetic energy).
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These coefficients can be determined by fitting a quadratic polynomial to
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electronic stopping data predicted by, for example, SRIM or TD-DFT. Multiple
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'Ecut c1 c2' values can be provided for multi-species simulations in the order
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of the atom types. There is an examples/USER/misc/electron_stopping/ directory,
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which illustrates uses of this command. Details of this implementation are
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further described in :ref:`Stewart2018 <Stewart2018>` and :ref:`Lee2020 <Lee2020>`.
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Restart, fix_modify, output, run start/stop, minimize info
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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No information about this fix is written to :doc:`binary restart files <restart>`.
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The :doc:`fix_modify <fix_modify>` options are not supported.
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This fix computes a global scalar, which can be accessed by various
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:doc:`output commands <Howto_output>`. The scalar is the total energy
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loss from electronic stopping applied by this fix since the start of
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the latest run. It is considered "intensive".
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The *start/stop* keywords of the :doc:`run <run>` command have no effect
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on this fix.
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Restrictions
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""""""""""""
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This pair style is part of the USER-MISC package. It is only enabled if
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LAMMPS was built with that package. See the :doc:`Build package <Build_package>`
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doc page for more info.
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Default
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"""""""
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The default is no limitation by region, and minneigh = 1.
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----------
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.. _elstopping:
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**(electronic stopping)** Wikipedia - Electronic Stopping Power: https://en.wikipedia.org/wiki/Stopping_power_%28particle_radiation%29
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.. _Nordlund98:
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**(Nordlund98)** Nordlund, Kai, et al. Physical Review B 57.13 (1998): 7556.
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.. _Nordlund95:
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**(Nordlund95)** Nordlund, Kai. Computational materials science 3.4 (1995): 448-456.
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.. _SRIM:
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**(SRIM)** SRIM webpage: http://www.srim.org/
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.. _CasP:
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**(CasP)** CasP webpage: https://www.helmholtz-berlin.de/people/gregor-schiwietz/casp_en.html
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.. _PASS:
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**(PASS)** PASS webpage: https://www.sdu.dk/en/DPASS
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.. _Stewart2018:
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**(Stewart2018)** J.A. Stewart, et al. (2018) Journal of Applied Physics, 123(16), 165902.
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.. _Lee2020:
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**(Lee2020)** C.W. Lee, et al. (2020) Physical Review B, 102(2), 024107.
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