# .param Files¶

Section author: Axel Huebl

Parameter files, *.param placed in include/picongpu/param/ are used to set all compile-time options for a PIConGPU simulation. This includes most fundamental options such as numerical solvers, floating precision, memory usage due to attributes and super-cell based algorithms, density profiles, initial conditions etc.

## Editing¶

For convenience, we provide a tool pic-edit to edit the compile-time input by its name. For example, if you want to edit the grid and time step resolution, file output and add a laser to the simulation, open the according files via:

# first switch to your input directory
cd \$HOME/picInputs/myLWFA

pic-edit grid fileOutput laser


See pic-edit --help for all available files:

Edit compile-time options for a PIConGPU input set

Opens .param files in an input set with the default "EDITOR".
If a .param file is not yet part of the input set but exists in the
defaults, it will be transparently added to the input set.

You must run this command inside an input directory.

The currently selected editor is: /usr/bin/vim.basic
You can change it via the "EDITOR" environment variable.

usage: pic-edit <input>

Available <input>s:
bremsstrahlung components density dimension fieldBackground fieldSolver fileOutput flylite grid ionizationEnergies ionizer isaac laser mallocMC memory particle particleCalorimeter particleFilters particleMerger physicalConstants pml png pngColorScales precision pusher radiation radiationObserver random species speciesAttributes speciesConstants speciesDefinition speciesInitialization starter synchrotronPhotons transitionRadiation unit


## Rationale¶

High-performance hardware comes with a lot of restrictions on how to use it, mainly memory, control flow and register limits. In order to create an efficient simulation, PIConGPU compiles to exactly the numerical solvers (kernels) and physical attributes (fields, species) for the setup you need to run, which will furthermore be specialized for a specific hardware.

This comes at a small cost: when even one of those settings is changed, you need to recompile. Nevertheless, wasting about 5 minutes compiling on a single node is nothing compared to the time you save at scale!

All options that are less or non-critical for runtime performance, such as specific ranges observables in plugins or how many nodes shall be used, can be set in run time configuration files (*.cfg) and do not need a recompile when changed.

## Files and Their Usage¶

If you use our pic-configure script wrappers, you do not need to set all available parameter files since we will add the missing ones with sane defaults. Those defaults are:

• a standard, single-precision, well normalized PIC cycle suitable for relativistic plasmas

• no external forces (no laser, no initial density profile, no background fields, etc.)

## All Files¶

When setting up a simulation, it is recommended to adjust .param files in the following order:

## Python Generator (Third party)¶

PoGit is a utility to generate a set of .param files and a .cfg file using a Pythonic API. PoGit is a third-party development and supports only a subset of PIConGPU compile- and run-time settings. However, the resulting output can serve as a basis to be edited as normal .param files.