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Merge branch 'add-error-explanations' of github.com:akohlmey/lammps into add-error-explanations

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Axel Kohlmeyer
2025-02-17 23:06:44 -05:00
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doc/src/Errors_details.rst
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@ -420,6 +420,48 @@ different arguments.
This error message might also require a close look at other LAMMPS input files
that are read in by the input script, such as data files or restart files.
.. _err0021:
Energy was not tallied on needed timestep (also virial, per-atom energy, per-atom virial)
-----------------------------------------------------------------------------------------
This error is generated when LAMMPS attempts to access an out-of-date
or non-existent energy, pressure, or virial.
For efficiency reasons,
LAMMPS does *not* calculate these quantities when the forces are calculated
on every timestep or iteration. Global quantities are only calculated
when they are needed for :doc:`thermo <thermo_style>` output (at the beginning, end,
and at regular intervals specified by the :doc:`thermo <thermo>` command). Similarly,
per-atom quantities are only calculated if they are needed to write per-atom
energy or virial to a dump file.
This system works fine for simple input scripts.
However, the many user-specified `variable`, `fix`, and `compute` commands
that LAMMPS provides make it difficult to anticipate when a quantity will
be requested. In some use cases, LAMMPS will figure out that a quantity is needed
and arrange for it to be calculated on that timestep e.g. if it is requested by
:doc:`fix ave/time <fix_ave_time>` or similar commands.
If that fails, it can be detected by a mismatch
between the current timestep and when a quantity was last calculated,
in which case an error message of this type is generated.
The most common cause of this type of error is requesting a quantity before
the start of the simulation.
.. code-block:: LAMMPS
# run 0 post no # this will fix the error
variable e equal pe # requesting energy compute
print "Potential energy = $e" # this will generate the error
run 1000 # start of simulation
This situation can be avoided by adding in a "run 0" command, as explained in
more detail in the "Variable Accuracy" section of the
:doc:`variable <variable>` doc page.
Another cause is requesting a quantity on a timestep that is not
a thermo or dump output timestep. This can often be
remedied by increasing the frequency of thermo or dump output.
.. _err0024:
Molecule topology/atom exceeds system topology/atom