.. _testing-and-validation-of-openmm:

Testing and Validation of OpenMM
################################

The goal of testing and validation is to make sure that OpenMM works correctly.
That means that it runs without crashing or otherwise failing, and that it
produces correct results.  Furthermore, it must work correctly on a variety of
hardware platforms (e.g. different models of GPU), software platforms (e.g.
operating systems and OpenCL implementations), and types of simulations.

OpenMM includes unit tests, which are small tests designed to test specific features
or pieces of code in isolation.  For example, a test of HarmonicBondForce might
create a System with just a few particles and bonds, compute the forces and
energy, and compare them to the analytically expected values.  There are
thousands of unit tests that collectively cover all of OpenMM.

In addition, a direct comparison has been made of the individual forces computed
by OpenMM to those computed by other programs for a collection of biomolecules.

Each of these types of tests is outlined in greater detail below; a discussion of the
current status of the tests is then given.


Description of Tests
********************


Unit tests
===========

The unit tests are with the source code, so if you build from source you can run
them yourself.  See Section :numref:`test-your-build` for details.  When you run the tests
(for example, by typing “make test” on Linux or Mac), it should produce output
something like this:
::

            Start   1: TestReferenceAndersenThermostat
      1/317 Test   #1: TestReferenceAndersenThermostat .............. Passed  0.26 sec
            Start   2: TestReferenceBrownianIntegrator
      2/317 Test   #2: TestReferenceBrownianIntegrator .............. Passed  0.13 sec
            Start   3: TestReferenceCheckpoints
      3/317 Test   #3: TestReferenceCheckpoints ..................... Passed  0.02 sec
      ... <many other tests> ...

Each line represents a test suite, which may contain multiple unit tests.  If
all tests within a suite passed, it prints the word “Passed” and how long the
suite took to execute.  Otherwise it prints an error message.  If any tests
failed, you can then run them individually (each one is a separate executable)
to get more details on what went wrong.


Direct comparisons between OpenMM and other programs
====================================================

As another check, identical systems are set up in OpenMM and in another program
(Gromacs 4.5 or Tinker 6.1), each one is used to compute the forces on atoms,
and the results are directly compared to each other.

Test Results
************

In this section, we highlight the major results obtained from the tests
described above.  They are not exhaustive, but should give a reasonable idea of
the level of accuracy you can expect from OpenMM.

Comparison to Reference Platform
================================

The differences between forces computed with the Reference platform and those
computed with the OpenCL or CUDA platform are shown in
:autonumref:`Table,force comparison between platforms`\ .  For every
atom, the relative difference between platforms was computed as
2·\|F\ :sub:`ref`\ –F\ :sub:`test`\ \|/(\|F\ :sub:`ref`\ \|+|F\ :sub:`test`\ \|), where
F\ :sub:`ref` is the force computed by the Reference platform and F\ :sub:`test`
is the force computed by the platform being tested (OpenCL or CUDA).  The median
over all atoms in a given system was computed to estimate the typical force
errors for that system.  Finally, the median of those values for all test
systems was computed to give the value shown in the table.

====================================  ========================  ====================  ===================  =====================
Force                                 OpenCL (single)           OpenCL (double)       CUDA (single)        CUDA (double)
====================================  ========================  ====================  ===================  =====================
Total Force                           2.53·10\ :sup:`-6`        1.44·10\ :sup:`-7`    2.56·10\ :sup:`-6`   8.78·10\ :sup:`-8`
HarmonicBondForce                     2.88·10\ :sup:`-6`        1.57·10\ :sup:`-13`   2.88·10\ :sup:`-6`   1.57·10\ :sup:`-13`
HarmonicAngleForce                    2.25·10\ :sup:`-5`        4.21·10\ :sup:`-7`    2.27·10\ :sup:`-5`   4.21·10\ :sup:`-7`
PeriodicTorsionForce                  8.23·10\ :sup:`-7`        2.44·10\ :sup:`-7`    9.27·10\ :sup:`-7`   2.56·10\ :sup:`-7`
RBTorsionForce                        4.86·10\ :sup:`-6`        1.46·10\ :sup:`-7`    4.72·10\ :sup:`-6`   1.4·10\ :sup:`-8`
NonbondedForce (no cutoff)            1.49·10\ :sup:`-6`        6.49·10\ :sup:`-8`    1.49·10\ :sup:`-6`   6.49·10\ :sup:`-8`
NonbondedForce (cutoff, nonperiodic)  9.74·10\ :sup:`-7`        4.88·10\ :sup:`-9`    9.73·10\ :sup:`-7`   4.88·10\ :sup:`-9`
NonbondedForce (cutoff, periodic)     9.82·10\ :sup:`-7`        4.88·10\ :sup:`-9`    9.8·10\ :sup:`-7`    4.88·10\ :sup:`-9`
NonbondedForce (Ewald)                1.33·10\ :sup:`-6`        5.22·10\ :sup:`-9`    1.33·10\ :sup:`-6`   5.22·10\ :sup:`-9`
NonbondedForce (PME)                  3.99·10\ :sup:`-5`        4.08·10\ :sup:`-6`    3.99·10\ :sup:`-5`   4.08·10\ :sup:`-6`
GBSAOBCForce (no cutoff)              3.0·10\ :sup:`-6`         1.76·10\ :sup:`-7`    3.09·10\ :sup:`-6`   9.4·10\ :sup:`-8`
GBSAOBCForce (cutoff, nonperiodic)    2.77·10\ :sup:`-6`        1.76·10\ :sup:`-7`    2.95·10\ :sup:`-6`   9.33·10\ :sup:`-8`
GBSAOBCForce (cutoff, periodic)       2.61·10\ :sup:`-6`        1.78·10\ :sup:`-7`    2.77·10\ :sup:`-6`   9.24·10\ :sup:`-8`
====================================  ========================  ====================  ===================  =====================

:autonumber:`Table,force comparison between platforms`\ :  Median relative difference in forces between Reference platform and
OpenCL/CUDA platform


Energy Conservation
===================

:autonumref:`Figure,energy drift` shows the total system energy versus time for three simulations of
ubiquitin in OBC implicit solvent.  All three simulations used the CUDA
platform, a Verlet integrator, a time step of 0.5 fs, no constraints, and no
cutoff on the nonbonded interactions.  They differ only in the level of numeric
precision that was used for calculations (see Chapter :numref:`platform-specific-properties`\ ).


.. figure:: ../../images/EnergyDrift.png
   :align: center

   :autonumber:`Figure,energy drift`: Total energy versus time for simulations run in three different
   precision modes.

For the mixed and double precision simulations, the drift in energy is almost
entirely diffusive with negligible systematic drift.  The single precision
simulation has a more significant upward drift with time, though the rate of
drift is still small compared to the rate of short term fluctuations.  Fitting a
straight line to each curve gives a long term rate of energy drift of 3.98
kJ/mole/ns for single precision, 0.217 kJ/mole/ns for mixed precision, and
0.00100 kJ/mole/ns for double precision.  In the more commonly reported units of
kT/ns/dof, these correspond to 4.3·10\ :sup:`-4` for single precision,
2.3·10\ :sup:`-5` for mixed precision, and 1.1·10\ :sup:`-7` for double precision.

Be aware that different simulation parameters will give different results.
These simulations were designed to minimize all sources of error except those
inherent in OpenMM.  There are other sources of error that may be significant in
other situations.  In particular:

* Using a larger time step increases the integration error (roughly
  proportional to *dt*\ :sup:`2`\ ).
* If a system involves constraints, the level of error will depend strongly on
  the constraint tolerance specified by the Integrator.
* When using Ewald summation or Particle Mesh Ewald, the accuracy will depend
  strongly on the Ewald error tolerance.
* Applying a distance cutoff to implicit solvent calculations will increase the
  error, and the shorter the cutoff is, the greater the error will be.


As a result, the rate of energy drift may be much greater in some simulations
than in the ones shown above.

Comparison to Gromacs
=====================

OpenMM and Gromacs 4.5.5 were each used to compute the atomic forces for
dihydrofolate reductase (DHFR) in implicit and explicit solvent.  The implicit
solvent calculations used the OBC solvent model and no cutoff on nonbonded
interactions.  The explicit solvent calculations used Particle Mesh Ewald and a
1 nm cutoff on direct space interactions.  For OpenMM, the Ewald error tolerance
was set to 10\ :sup:`-6`\ .  For Gromacs, :code:`fourierspacing` was set to
0.07 and :code:`ewald_rtol` to 10\ :sup:`-6`\ .  No constraints were applied
to any degrees of freedom.  Both programs used single precision.  The test was
repeated for OpenCL, CUDA, and CPU platforms.

For every atom, the relative difference between OpenMM and Gromacs was computed
as 2·\|F\ :sub:`MM`\ –F\ :sub:`Gro`\ \|/(\|F\ :sub:`MM`\ \|+\|F\ :sub:`Gro`\ \|),
where F\ :sub:`MM` is the force computed by OpenMM and F\ :sub:`Gro` is the
force computed by Gromacs.  The median over all atoms is shown in :autonumref:`Table,comparison to Gromacs`\ .

=============   ===================  ===================  ===================
Solvent Model   OpenCL               CUDA                 CPU
=============   ===================  ===================  ===================
Implicit        7.66·10\ :sup:`-6`   7.68·10\ :sup:`-6`   1.94·10\ :sup:`-5`
Explicit        6.77·10\ :sup:`-5`   6.78·10\ :sup:`-5`   9.89·10\ :sup:`-5`
=============   ===================  ===================  ===================

:autonumber:`Table,comparison to Gromacs`\ :  Median relative difference in forces between OpenMM and Gromacs

