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docs: start with howto section

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Richard Berger
2024-08-18 07:59:29 -06:00
parent 0b54a0b1e3
commit b7cf79a7b9
5 changed files with 34 additions and 28 deletions
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2
doc/src/Howto_couple.rst
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@ -36,7 +36,7 @@ the context of your application.
steps, invoke the command, etc.
In this scenario, the other code can be called as a library, as in
1., or it could be a stand-alone code, invoked by a system() call
1., or it could be a stand-alone code, invoked by a ``system()`` call
made by the command (assuming your parallel machine allows one or
more processors to start up another program). In the latter case the
stand-alone code could communicate with LAMMPS through files that the

30
doc/src/Howto_elastic.rst
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@ -4,21 +4,27 @@ Calculate elastic constants
Elastic constants characterize the stiffness of a material. The formal
definition is provided by the linear relation that holds between the
stress and strain tensors in the limit of infinitesimal deformation.
In tensor notation, this is expressed as s_ij = C_ijkl \* e_kl, where
the repeated indices imply summation. s_ij are the elements of the
symmetric stress tensor. e_kl are the elements of the symmetric strain
tensor. C_ijkl are the elements of the fourth rank tensor of elastic
constants. In three dimensions, this tensor has 3\^4=81 elements. Using
Voigt notation, the tensor can be written as a 6x6 matrix, where C_ij
is now the derivative of s_i w.r.t. e_j. Because s_i is itself a
derivative w.r.t. e_i, it follows that C_ij is also symmetric, with at
most 7\*6/2 = 21 distinct elements.
In tensor notation, this is expressed as
.. math::
s_{ij} = C_{ijkl} e_{kl}
where
the repeated indices imply summation. :math:`s_{ij}` are the elements of the
symmetric stress tensor. :math:`e_{kl}` are the elements of the symmetric strain
tensor. :math:`C_{ijkl}` are the elements of the fourth rank tensor of elastic
constants. In three dimensions, this tensor has :math:`3^4=81` elements. Using
Voigt notation, the tensor can be written as a 6x6 matrix, where :math:`C_{ij}`
is now the derivative of :math:`s_i` w.r.t. :math:`e_j`. Because :math:`s_i` is itself a
derivative w.r.t. :math:`e_i`, it follows that :math:`C_{ij}` is also symmetric, with at
most :math:`\frac{7 \times 6}{2}` = 21 distinct elements.
At zero temperature, it is easy to estimate these derivatives by
deforming the simulation box in one of the six directions using the
:doc:`change_box <change_box>` command and measuring the change in the
stress tensor. A general-purpose script that does this is given in the
examples/ELASTIC directory described on the :doc:`Examples <Examples>`
``examples/ELASTIC`` directory described on the :doc:`Examples <Examples>`
doc page.
Calculating elastic constants at finite temperature is more
@ -33,7 +39,7 @@ the :doc:`compute born/matrix <compute_born_matrix>` command,
which works for any bonded or non-bonded potential in LAMMPS.
The most expensive part of the calculation is the sampling of
the stress fluctuations. Several examples of this method are
provided in the examples/ELASTIC_T/BORN_MATRIX directory
provided in the ``examples/ELASTIC_T/BORN_MATRIX`` directory
described on the :doc:`Examples <Examples>` doc page.
A second way is to measure
@ -43,7 +49,7 @@ the systematic and statistical errors in this method, the magnitude of
the deformation must be chosen judiciously, and care must be taken to
fully equilibrate the deformed cell before sampling the stress
tensor. An example of this method is provided in the
examples/ELASTIC_T/DEFORMATION directory
``examples/ELASTIC_T/DEFORMATION`` directory
described on the :doc:`Examples <Examples>` doc page.
Another approach is to sample the triclinic cell fluctuations

10
doc/src/Howto_multiple.rst
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@ -44,7 +44,7 @@ For large numbers of independent simulations, you can use
:doc:`variables <variable>` and the :doc:`next <next>` and
:doc:`jump <jump>` commands to loop over the same input script
multiple times with different settings. For example, this
script, named in.polymer
script, named ``in.polymer``
.. code-block:: LAMMPS
@ -57,7 +57,7 @@ script, named in.polymer
next d
jump in.polymer
would run 8 simulations in different directories, using a data.polymer
would run 8 simulations in different directories, using a ``data.polymer``
file in each directory. The same concept could be used to run the
same system at 8 different temperatures, using a temperature variable
and storing the output in different log and dump files, for example
@ -83,10 +83,10 @@ partition of processors. LAMMPS can be run on multiple partitions via
the :doc:`-partition command-line switch <Run_options>`.
In the last 2 examples, if LAMMPS were run on 3 partitions, the same
scripts could be used if the "index" and "loop" variables were
scripts could be used if the ``index`` and ``loop`` variables were
replaced with *universe*\ -style variables, as described in the
:doc:`variable <variable>` command. Also, the "next t" and "next a"
commands would need to be replaced with a single "next a t" command.
:doc:`variable <variable>` command. Also, the :lammps:`next t` and :lammps:`next a`
commands would need to be replaced with a single :lammps:`next a t` command.
With these modifications, the 8 simulations of each script would run
on the 3 partitions one after the other until all were finished.
Initially, 3 simulations would be started simultaneously, one on each

18
doc/src/Howto_restart.rst
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@ -26,8 +26,8 @@ scripts are based on. If that script had the line
restart 50 tmp.restart
added to it, it would produce 2 binary restart files (tmp.restart.50
and tmp.restart.100) as it ran.
added to it, it would produce two binary restart files (``tmp.restart.50``
and ``tmp.restart.100``) as it ran.
This script could be used to read the first restart file and re-run the
last 50 timesteps:
@ -47,21 +47,21 @@ last 50 timesteps:
run 50
Note that the following commands do not need to be repeated because
their settings are included in the restart file: *units, atom_style,
special_bonds, pair_style, bond_style*. However, these commands do
their settings are included in the restart file: :lammps:`units`, :lammps:`atom_style`,
:lammps:`special_bonds`, :lammps:`pair_style`, :lammps:`bond_style`. However, these commands do
need to be used, since their settings are not in the restart file:
*neighbor, fix, timestep*\ .
:lammps:`neighbor`, :lammps:`fix`, :lammps:`timestep`.
If you actually use this script to perform a restarted run, you will
notice that the thermodynamic data match at step 50 (if you also put a
"thermo 50" command in the original script), but do not match at step
:lammps:`thermo 50` command in the original script), but do not match at step
100. This is because the :doc:`fix langevin <fix_langevin>` command
uses random numbers in a way that does not allow for perfect restarts.
As an alternate approach, the restart file could be converted to a data
file as follows:
.. code-block:: LAMMPS
.. code-block:: bash
lmp_g++ -r tmp.restart.50 tmp.restart.data
@ -89,8 +89,8 @@ Then, this script could be used to re-run the last 50 steps:
reset_timestep 50
run 50
Note that nearly all the settings specified in the original *in.chain*
script must be repeated, except the *pair_coeff* and *bond_coeff*
Note that nearly all the settings specified in the original ``in.chain``
script must be repeated, except the :lammps:`pair_coeff` and :lammps:`bond_coeff`
commands, since the new data file lists the force field coefficients.
Also, the :doc:`reset_timestep <reset_timestep>` command is used to tell
LAMMPS the current timestep. This value is stored in restart files, but

2
doc/src/Howto_viz.rst
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@ -6,7 +6,7 @@ analyzed in a variety of ways.
LAMMPS snapshots are created by the :doc:`dump <dump>` command, which
can create files in several formats. The native LAMMPS dump format is a
text file (see "dump atom" or "dump custom") which can be visualized by
text file (see :lammps:`dump atom` or :lammps:`dump custom`) which can be visualized by
`several visualization tools <https://www.lammps.org/viz.html>`_ for MD
simulation trajectories. `OVITO <https://www.ovito.org>`_ and `VMD
<https://www.ks.uiuc.edu/Research/vmd>`_ seem to be the most popular