Not a good example.

examples/HelloWorld.cpp

-// Suppress irrelevant warnings from Microsoft's compiler.
-#ifdef _MSC_VER
-    #pragma warning(disable:4996)   // sprintf is unsafe 
-    #pragma warning(disable:4251)   // no dll interface for some classes
-#endif
-
-#include "OpenMM.h"
-
-#include <iostream>
-#include <string>
-#include <limits>
-
-using namespace OpenMM;
-
-Vec3 operator*(const Vec3& v, double r) {
-    return Vec3(v[0]*r, v[1]*r, v[2]*r);
-}
-
-Vec3 operator*(double r, const Vec3& v) {
-    return Vec3(r*v[0], r*v[1], r*v[2]);
-}
-
-// This is not part of OpenMM; just a struct we can use to collect
-// atom parameters for this example. Normally atom parameters would
-// come from the force field's parameterization file.
-// We're going to use data in Angstrom and Kilocalorie units and
-// show how to safely convert to OpenMM's internal unit system
-// which uses nanometers and kilojoules.
-struct AtomInfo {
-    char*  symbol;
-    double mass, charge, vdwRadiusAng, vdwEnergyKcal;
-    Vec3   startPosAng;
-};
-
-static AtomInfo atoms[] = {
-    {"Na+", 22.99,  1, 1.8680, 0.00277, Vec3(-3,0,0)},
-    {"Cl-", 35.45, -1, 2.4700, 0.1000,  Vec3( 3,0,0)},
-    {""} // end of list
-};
-
-static const double Temperature         = 100;   // Kelvins
-static const double FrictionInPerPs     = 91.;   // collisions per ps
-static const double StepSizeFs          = 2;     // femtoseconds
-static const double ReportIntervalFs    = 1000;
-static const double SimulationTimePs    = 1000;  // total simulation time (ps)
-
-static const double SigmaPerVdwRadius = 2*std::pow(2., -1./6.);
-
-static double showState(const OpenMMContext&);
-
-int main() {
-try {
-    // Load all available OpenMM plugins from their default location.
-    Platform::loadPluginsFromDirectory(Platform::getDefaultPluginsDirectory());
-
-    // Create a System and a NonbondedForce object within the System. 
-    System system;
-    NonbondedForce* nonbond = new NonbondedForce();
-    system.addForce(nonbond);
-
-    int numAtoms = 0;
-    for (; *atoms[numAtoms].symbol; ++numAtoms) {
-        const AtomInfo& atom = atoms[numAtoms];
-        system.addParticle(atom.mass);
-        nonbond->addParticle(atom.charge,
-                             atom.vdwRadiusAng  * NmPerAngstrom * SigmaPerVdwRadius,
-                             atom.vdwEnergyKcal * KJPerKcal);
-    }
-
-    // Create an integrator object for advancing time.
-    //LangevinIntegrator integrator(Temperature, Friction, StepSizeFs * PsPerFs);
-    VerletIntegrator integrator(StepSizeFs * PsPerFs);
-
-    // Create an OpenMM Context for execution; let it choose best platform.
-    OpenMMContext context(system, integrator);
-
-    std::cout << "Will use OpenMM platform " << context.getPlatform().getName() << std::endl;
-    std::cout << "eps(float)=" << std::numeric_limits<float>::epsilon() << std::endl;
-
-    // Fill in a vector of starting positions, one per atom.
-    std::vector<Vec3> positions(numAtoms);
-    for (int i=0; i < numAtoms; ++i)
-        positions[i] = atoms[i].startPosAng * NmPerAngstrom;
-
-    // Set the starting positions in the OpenMM context. Velocities will be zero.
-    context.setPositions(positions);
-
-    // Output the initial state.
-    double time = showState(context);
-
-    const int NumSilentSteps = (int)(ReportIntervalFs / StepSizeFs + 0.5);
-    do {
-        integrator.step(NumSilentSteps);
-        time = showState(context);
-    } while (time < SimulationTimePs);
-
-    } catch(const std::exception& e) {
-        std::cout << "EXCEPTION: " << e.what() << std::endl;
-        return 1;
-    }
-
-    return 0;
-}
-
-static double
-showState(const OpenMMContext& context) {
-    // Caution: at the moment asking for energy requires use of slow reference calculation.
-    const State                 state       = context.getState(  State::Positions | State::Velocities 
-                                                               | State::Forces    | State::Energy);
-    const double                energy      = state.getPotentialEnergy() + state.getKineticEnergy();
-    const std::vector<Vec3>&    positions   = state.getPositions();
-    const std::vector<Vec3>&    forces      = state.getForces();
-
-    std::cout << "t=" << state.getTime() << " E=" << energy << std::endl;
-    const double dt = StepSizeFs*PsPerFs;
-    for (unsigned i=0; i < positions.size(); ++i) {
-        const Vec3 dq = dt*dt/(2*atoms[i].mass) * forces[i];
-        std::cout << i << " " << positions[i] * AngstromsPerNm << std::endl;
-        std::cout << "f=" << forces[i] << " dq=" << dq << std::endl;
-    }
-
-    return state.getTime();
-}
-