## Basics of Using LAMMPS

Return to top-level of LAMMPS documentation.

- Distribution
- Making LAMMPS
- Running LAMMPS
- Examples
- Other Tools
- Extending LAMMPS

### Distribution

When you unzip/untar the LAMMPS distribution you should have several directories:

- src = source files for LAMMPS
- doc = HTML documentation
- examples = sample problems with inputs and outputs
- tools = serial program for creating and massaging LAMMPS data files
- converters = msi2lmp, lmp2arc, amber = codes & scripts for converting between MSI/Discover, AMBER, and LAMMPS formats

### Making LAMMPS

The src directory contains the F90 and C source files for LAMMPS as well as several sample Makefiles for different machines. To make LAMMPS for a specific machine, you simply type

make machine

from within the src directoy. E.g. "make sgi" or "make t3e". This should create an executable such as lmp_sgi or lmp_t3e. For optimal performance you'll want to use a good F90 compiler to make LAMMPS; on Linux boxes I've been told the Leahy F90 compiler is a good choice. (If you don't have an F90 compiler, I can give you an older F77-based version of LAMMPS 99, but you'll lose the dynamic memory and some other new features in LAMMPS 2001.)

In the src directory, there is one top-level Makefile and several low-level machine-specific files named Makefile.xxx where xxx = the machine name. If a low-level Makefile exists for your platform, you do not need to edit the top-level Makefile. However you should check the system-specific section of the low-level Makefile to ensure the various paths are correct for your environment. If a low-level Makefile does not exist for your platform, you will need to add a suitable target to the top-level Makefile. You will also need to create a new low-level Makefile using one of the existing ones as a template. If you wish to make LAMMPS for a single-processor workstation that doesn't have an installed MPI library, you can specify the "serial" target which uses a directory of MPI stubs to link against - e.g. "make serial". You will need to make the stub library (type "make" in STUBS directory) for your workstation before doing this.

Note that the two-level Makefile system allows you to make LAMMPS for multiple platforms. Each target creates its own object directory for separate storage of its *.o files.

There are a few compiler switches of interest which can be specified in the low-level Makefiles. If you use a F90FLAGS switch of -DSYNC then synchronization calls will be made before the timing routines in integrate.f. This may slow down the code slightly, but will make the individual timings reported at the end of a run more accurate. The F90FLAGS setting of -DSENDRECV will use MPI_Sendrecv calls for data exchange between processors instead of MPI_Irecv, MPI_Send, MPI_Wait. Sendrecv is often slower, but on some platforms can be faster, so it is worth trying, particularly if your communication timings seem slow.

The CCFLAGS setting in the low-level Makefiles requires a FFT setting, for example -DFFT_SGI or -DFFT_T3E. This is for inclusion of the appropriate machine-specific native 1-d FFT libraries on various platforms. Currently, the supported machines and switches (used in fft_3d.c) are FFT_SGI, FFT_DEC, FFT_INTEL, FFT_T3E, and FFT_FFTW. The latter is a publicly available portable FFT library, FFTW, which you can install on any machine. If none of these options is suitable for your machine, please contact me, and we'll discuss how to add the capability to call your machine's native FFT library. You can also use FFT_NONE if you have no need to use the PPPM option in LAMMPS.

For Linux and T3E compilation, there is a also a CCFLAGS setting for KLUDGE needed (see Makefile.linux and Makefile.t3e). This is to enable F90 to call C with appropriate underscores added to C function names.

### Running LAMMPS

LAMMPS is run by redirecting a text file (script) of input commands into it.

lmp_sgi < in.lj

lmp_t3e < in.lj

The script file contains commands that specify the parameters for the simulation as well as to read other necessary files such as a data file that describes the initial atom positions, molecular topology, and force-field parameters. The input_commands page describes all the possible commands that can be used. The data_format page describes the format of the data file.

LAMMPS can be run on any number of processors, including a single processor. In principle you should get identical answers on any number of processors and on any machine. In practice, numerical round-off can cause slight differences and eventual divergence of dynamical trajectories.

When LAMMPS runs, it estimates the array sizes it should allocate based on the problem you are simulating and the number of processors you are running on. If you run out of physical memory, you will get a F90 allocation error and the code should hang or crash. The only thing you can do about this is run on more processors or run a smaller problem. If you get an error message to the screen about "boosting" something, it means LAMMPS under-estimated the size needed for one (or more) data arrays. The "extra memory" command can be used in the input script to augment these sizes at run time. A few arrays are hard-wired to sizes that should be sufficient for most users. These are specified with parameter settings in the global.f file. If you get a message to "boost" one of these parameters you will have to change it and re-compile LAMMPS.

Some LAMMPS errors are detected at setup; others like neighbor list overflow may not occur until the middle of a run. Except for F90 allocation errors which may cause the code to hang (with an error message) since only one processor may incur the error, LAMMPS should always print a message to the screen and exit gracefully when it encounters a fatal error. If the code ever crashes or hangs without spitting out an error message first, it's probably a bug, so let me know about it. Of course this applies to algorithmic or parallelism issues, not to physics mistakes, like specifying too big a timestep or putting 2 atoms on top of each other! One exception is that different MPI implementations handle buffering of messages differently. If the code hangs without an error message, it may be that you need to specify an MPI setting or two (usually via an environment variable) to enable buffering or boost the sizes of messages that can be buffered.

### Examples

There are several directories of sample problems in the examples directory. All of them use an input file (in.*) of commands and a data file (data.*) of initial atomic coordinates and produce one or more output files. Sample outputs on different machines and numbers of processors are included to compare your answers to. See the README file in the examples sub-directory for more information on what LAMMPS features the examples illustrate.

(1) lj = atomic simulations of Lennard-Jones systems.

(2) class2 = phenyalanine molecule using the DISCOVER cff95 class 2 force field.

(3) lc = liquid crystal molecules with various Coulombic options and periodicity settings.

(4) flow = 2d flow of Lennard-Jones atoms in a channel using various constraint options.

(5) polymer = bead-spring polymer models with one or two chain types.

### Other Tools

The converters directory has source code and scripts for tools that perform input/output file conversions between MSI Discover, AMBER, and LAMMPS formats. See the README files for the individual tools for additional information.

The tools directory has several serial programs that create and massage LAMMPS data files.

(1) setup_chain.f = create a data file of polymer bead-spring chains

(2) setup_lj.f = create a data file of an atomic LJ mixture of species

(3) setup_flow_2d.f = create a 2d data file of LJ particles with walls for a flow simulation

(4) replicate.c = replicate or scale an existing data file into a new one

(5) peek_restart.f = print-out info from a binary LAMMPS restart file

(6) restart2data.f = convert a binary LAMMPS restart file into a text data file

See the comments at the top of each source file for information on how to use the tool.

### Extending LAMMPS

User-written routines can be compiled and linked with LAMMPS, then invoked with the "diagnostic" command as LAMMPS runs. These routines can be used for on-the-fly diagnostics or a variety of other purposes. The examples/lc directory shows an example of using the diagnostic command with the in.lc.big.fixes input script. A sample diagnostic routine is given there also: diagnostic_temp_molecules.f.