Created OPENMM_DEFAULT_PLATFORM environment variable

docs-source/usersguide/application.rst

@@ -675,11 +675,17 @@ Platforms
 
 When creating a :class:`Simulation`, you can optionally tell it what :class:`Platform` to use.
 OpenMM includes four platforms: :class:`Reference`, :class:`CPU`, :class:`CUDA`, and :class:`OpenCL`.  For a
-description of the differences between them, see Section :ref:`platforms`.  If you do not
-specify a :class:`Platform`, it will select one automatically.  Usually its choice will
-be reasonable, but you may want to change it.
+description of the differences between them, see Section :ref:`platforms`.  There are three ways in which
+the :class:`Platform` can be chosen:
 
-The following lines specify to use the :class:`CUDA` platform:
+1. By default, OpenMM will try to select the fastest available :class:`Platform`.  Usually its choice will
+be reasonable, but sometimes you may want to change it.
+
+2. Alternatively, you can set the :envvar:`OPENMM_DEFAULT_PLATFORM` environment variable to the name
+of the :class:`Platform` to use.  This overrides the default logic.
+
+3. Finally, you can explicitly specify a :class:`Platform` object in your script when you create the
+:class:`Simulation`.  The following lines specify to use the :class:`CUDA` platform:
 ::
 
     platform = Platform.getPlatformByName('CUDA')

openmmapi/src/ContextImpl.cpp

@@ -41,6 +41,7 @@
 #include "openmm/Context.h"
 #include <algorithm>
 #include <cmath>
+#include <cstdlib>
 #include <iostream>
 #include <map>
 #include <utility>
@@ -115,6 +116,11 @@ ContextImpl::ContextImpl(Context& owner, const System& system, Integrator& integ
     // Select a platform to use.
     
     vector<pair<double, Platform*> > candidatePlatforms;
+    if (platform == NULL) {
+        char* defaultPlatform = getenv("OPENMM_DEFAULT_PLATFORM");
+        if (defaultPlatform != NULL)
+            platform = &Platform::getPlatformByName(string(defaultPlatform));
+    }
     if (platform == NULL) {
         for (int i = 0; i < Platform::getNumPlatforms(); i++) {
             Platform& p = Platform::getPlatform(i);