# Physical validation¶

Physical validation tests check whether simulation results correspond to physical (or mathematical) expectations.

Unlike the existing tests, we are not be able to keep these tests in the “seconds, not minutes” time frame, rather aiming for “hours, not days”. They should therefore be ran periodically, but probably not for every build.

Also, given the long run time, it will in many cases be necessary to separate running of the systems (e.g. to run it at a specific time, or on a different resource), such that the make script does give the option to

• prepare run files and an execution script,
• or prepare, run and analyze in one go.

## Test description¶

Currently, simulation results are tested against three physically / mathematically expected results:

• Integrator convergence: A symplectic integrator can be shown to conserve a constant of motion (such as the energy in a micro-canonical simulation) up to a fluctuation that is quadratic in time step chosen. Comparing two or more constant-of-motion trajectories realized using different time steps (but otherwise unchanged simulation parameters) allows a check of the symplecticity of the integration. Note that lack of symplecticity does not necessarily imply an error in the integration algorithm, it can also hint at physical violations in other parts of the model, such as non-continuous potential functions, imprecise handling of constraints, etc.
• Kinetic energy distribution: The kinetic energy trajectory of a (equilibrated) system sampling a canonical or an isothermal-isobaric ensemble is expected to be Maxwell-Boltzmann distributed. The similarity between the physically expected and the observed distribution allows to validate the sampled kinetic energy ensemble.
• Distribution of configurational quantities: As the distribution of configurational quantities like the potential energy or the volume are in general not known analytically, testing the likelihood of a trajectory sampling a given ensemble is less straightforward than for the kinetic energy. However, generally, the ratio of the probability distribution between samples of the same ensemble at different state points (e.g. at different temperatures, different pressures) is known. Comparing two simulations at different state points therefore allows a validation of the sampled ensemble.

The physical validation included in GROMACS tests a range of the most-used settings on several systems. The general philosophy is to leave most settings to default values with the exception of the ones explicitly tested in order to be sensitive to changes in the default values. The test set will be enlarged as we discover interesting test systems and corner cases. Under double precision, some additional tests are ran, and some other tests are ran using a lower tolerance.

### Integrator convergence¶

All simulations performed under NVE on Argon (1000 atoms) and water (900 molecules) systems. As these tests are very sensitive to numerical imprecision, they are performed with long-range corrections for both Lennard-Jones and electrostatic interactions, with a very low pair-list tolerance (verlet-buffer-tolerance = 1e-10), and high LINCS settings where applicable.

Argon:

• Integrators: - integrator = md - integrator = md-vv
• Long-range corrections LJ: - vdwtype = PME - vdwtype = cut-off, vdw-modifier = force-switch, rvdw-switch = 0.8

Water:

• Integrators: - integrator = md - integrator = md-vv
• Long-range corrections LJ: - vdwtype = PME - vdwtype = cut-off, vdw-modifier = force-switch, rvdw-switch = 0.8
• Long-range corrections electrostatics: - coulombtype = PME, fourierspacing = 0.05
• Constraint algorithms: - constraint-algorithm = lincs, lincs-order = 6, lincs-iter = 2 - constraint-algorithm = none - SETTLE

### Ensemble tests¶

The generated ensembles are tested with Argon (1000 atoms) and water (900 molecules, with SETTLE and PME) systems, in the following combinations:

• integrator = md, tcoupl = v-rescale, tau-t = 0.1, ref-t = 87.0 (Argon) or ref-t = 298.15 (Water)
• integrator = md, tcoupl = v-rescale, tau-t = 0.1, ref-t = 87.0 (Argon) or ref-t = 298.15 (Water), pcoupl = parrinello-rahman, ref-p = 1.0, compressibility = 4.5e-5
• integrator = md-vv, tcoupl = v-rescale, tau-t = 0.1, ref-t = 87.0 (Argon) or ref-t = 298.15 (Water)
• integrator = md-vv, tcoupl = nose-hoover, tau-t = 1.0, ref-t = 87.0 (Argon) or ref-t = 298.15 (Water), pcoupl = mttk, ref-p = 1.0, compressibility = 4.5e-5

All thermostats are applied to the entire system (tc-grps = system). The simulations run for 1ns at 2fs time step with Verlet cut-off. All other settings left to default values.

## Building and testing using the build system¶

Since these tests can not be ran at the same frequency as the current tests, they are kept strictly opt-in via -DGMX_PHYSICAL_VALIDATION=ON, with -DGMX_PHYSICAL_VALIDATION=OFF being the default. Independently of that, all previously existing build targets are unchanged, including make check.

If physical validation is turned on, a number of additional make targets can be used:

• make check is unchanged, it builds the main binaries and the unit tests, then runs the unit tests and, if available, the regression tests.
• make check-phys builds the main binaries, then runs the physical validation tests. Warning: This requires to simulate all systems and might take several hours on a average machine!
• make check-all combines make check and make check-phys.

As the simulations needed to perform the physical validation tests may take long, it might be advantageous to run them on an external resource. To enable this, two additional make targets are present:

• make check-phys-prepare prepares all simulation files under tests/physicalvalidation of the build directory, as well as a rudimentary run script in the same directory.
• make check-phys-analyze runs the same tests as make check-phys, but does not simulate the systems. Instead, this target assumes that the results can be found under tests/physicalvalidation of the build directory.

The intended usage of these additional targets is to prepare the simulation files, then run them on a different resource or at a different time, and later analyze them. If you want to use this, be aware (i) that the run script generated is very simple and might need (considerable) tuning to work with your setup, and (ii) that the analysis script is sensitive to the folder structure, so make sure to preserve it when copying the results to / from another resource.

Additionally to the mentioned make targets, a number of internal make targets are defined. These are not intended to be used directly, but are necessary to support the functionality described above, especially the complex dependencies. These internal targets include run-ctest, run-ctest-nophys, run-ctest-phys and run-ctest-phys-analyze running the different tests, run-physval-sims running the simulations for physical validation, and missing-tests-notice, missing-tests-notice-all, missing-phys-val-phys, missing-phys-val-phys-analyze and missing-phys-val-all notifying users about missing tests.

## Direct usage of the python script¶

The make commands mentioned above are calling the python script tests/physicalvalidation/gmx_physicalvalidation.py, which can be used independently of the make system. Use the -h flag for the general usage information, and the --tests for more details on the available physical validations.

The script requires a json file defining the tests as an input. Among other options, it allows to define the GROMACS binary and the working directory to be used, and to decide whether to only prepare the simulations, prepare and run the simulations, only analyze the simulations, or do all three steps at once.

The available tests are listed in the systems.json (tests standardly used for single precision builds) and systems_d.json (tests standardly used for double precision builds) files in the same directory, the GROMACS files are in the folder systems/.
The json files lists the different test. Each test has a "name" attribute, which needs to be unique, a "dir" attribute, which denotes the directory of the system (inside the systems/ directory) to be tested, and a "test" attribute which lists the validations to be performed on the system. Additionally, the optional "grompp_args" and "mdrun_args" attributes allow to pass specific arguments to gmx grompp or gmx mdrun, respectively. A single test can contain several validations, and several independent tests can be performed on the same input files.
To add a new test to a present system, add the test name and the arguments to the json file(s). To use a new system, add a subfolder in the systems/ directory containing input/system.{gro,mdp,top} files defining your system.