Faster algorithm for DrudeSCFIntegrator (#4354)

* Faster algorithm for DrudeSCFIntegrator

* Minor code simplification

* Reduced default error tolerance

* Fixed errors on NVIDIA

plugins/drude/openmmapi/src/DrudeSCFIntegrator.cpp

@@ -6,7 +6,7 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2008-2013 Stanford University and the Authors.      *
+ * Portions copyright (c) 2008-2023 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -46,7 +46,7 @@ DrudeSCFIntegrator::DrudeSCFIntegrator(double stepSize) : DrudeIntegrator(stepSi
 {
     setDrudeTemperature(0.0);  // This is only used to initialize velocities for this integrator
     setStepSize(stepSize);
-    setMinimizationErrorTolerance(0.1);
+    setMinimizationErrorTolerance(1.0);
     setConstraintTolerance(1e-5);
     setMaxDrudeDistance(0.0);
 }

plugins/drude/platforms/common/src/CommonDrudeKernels.cpp

@@ -6,7 +6,7 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2013-2020 Stanford University and the Authors.      *
+ * Portions copyright (c) 2013-2023 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -375,36 +375,43 @@ double CommonIntegrateDrudeLangevinStepKernel::computeKineticEnergy(ContextImpl&
 }
 
 CommonIntegrateDrudeSCFStepKernel::~CommonIntegrateDrudeSCFStepKernel() {
-    if (minimizerPos != NULL)
-        lbfgs_free(minimizerPos);
 }
 
 void CommonIntegrateDrudeSCFStepKernel::initialize(const System& system, const DrudeSCFIntegrator& integrator, const DrudeForce& force) {
     cc.initializeContexts();
     ContextSelector selector(cc);
-
-    // Identify Drude particles.
-    
-    for (int i = 0; i < force.getNumParticles(); i++) {
+    int numDrude = force.getNumParticles();
+    drudeParams.initialize<mm_float4>(cc, numDrude, "drudeParams");
+    drudeIndices.initialize<int>(cc, numDrude, "drudeIndices");
+    drudeParents.initialize<mm_int4>(cc, numDrude, "drudeParents");
+    vector<mm_float4> paramVec(numDrude);
+    vector<mm_int4> parentVec(numDrude);
+    drudeIndexVec.resize(numDrude);
+    for (int i = 0; i < numDrude; i++) {
         int p, p1, p2, p3, p4;
         double charge, polarizability, aniso12, aniso34;
         force.getParticleParameters(i, p, p1, p2, p3, p4, charge, polarizability, aniso12, aniso34);
-        drudeParticles.push_back(p);
+        double a1 = (p2 == -1 ? 1 : aniso12);
+        double a2 = (p3 == -1 || p4 == -1 ? 1 : aniso34);
+        double a3 = 3-a1-a2;
+        double k3 = ONE_4PI_EPS0*charge*charge/(polarizability*a3);
+        double k1 = ONE_4PI_EPS0*charge*charge/(polarizability*a1) - k3;
+        double k2 = ONE_4PI_EPS0*charge*charge/(polarizability*a2) - k3;
+        paramVec[i] = mm_float4((float) k1, (float) k2, (float) k3, 0.0f);
+        drudeIndexVec[i] = p;
+        parentVec[i] = mm_int4(p1, p2, p3, p4);
     }
-    
-    // Initialize the energy minimizer.
-    
-    minimizerPos = lbfgs_malloc(drudeParticles.size()*3);
-    if (minimizerPos == NULL)
-        throw OpenMMException("DrudeSCFIntegrator: Failed to allocate memory");
-    lbfgs_parameter_init(&minimizerParams);
-    minimizerParams.linesearch = LBFGS_LINESEARCH_BACKTRACKING_STRONG_WOLFE;    
+    drudeParams.upload(paramVec);
+    drudeIndices.upload(drudeIndexVec);
+    drudeParents.upload(parentVec);
 
     // Create the kernels.
     
     ComputeProgram program = cc.compileProgram(CommonKernelSources::verlet);
     kernel1 = program->createKernel("integrateVerletPart1");
     kernel2 = program->createKernel("integrateVerletPart2");
+    program = cc.compileProgram(CommonDrudeKernelSources::drudeSCF);
+    minimizeKernel = program->createKernel("minimizeDrudePositions");
     prevStepSize = -1.0;
 }
 
@@ -431,6 +438,14 @@ void CommonIntegrateDrudeSCFStepKernel::execute(ContextImpl& context, const Drud
         kernel2->addArg(integration.getPosDelta());
         if (cc.getUseMixedPrecision())
             kernel2->addArg(cc.getPosqCorrection());
+        minimizeKernel->addArg((int) drudeParams.getSize());
+        minimizeKernel->addArg(cc.getPaddedNumAtoms());
+        minimizeKernel->addArg();
+        minimizeKernel->addArg(cc.getPosq());
+        minimizeKernel->addArg(cc.getLongForceBuffer());
+        minimizeKernel->addArg(drudeParams);
+        minimizeKernel->addArg(drudeIndices);
+        minimizeKernel->addArg(drudeParents);
     }
     if (dt != prevStepSize) {
         if (cc.getUseDoublePrecision() || cc.getUseMixedPrecision()) {
@@ -480,96 +495,24 @@ double CommonIntegrateDrudeSCFStepKernel::computeKineticEnergy(ContextImpl& cont
     return cc.getIntegrationUtilities().computeKineticEnergy(0.5*integrator.getStepSize());
 }
 
-struct MinimizerData {
-    ContextImpl& context;
-    ComputeContext& cc;
-    vector<int>& drudeParticles;
-    MinimizerData(ContextImpl& context, ComputeContext& cc, vector<int>& drudeParticles) : context(context), cc(cc), drudeParticles(drudeParticles) {}
-};
-
-static lbfgsfloatval_t evaluate(void *instance, const lbfgsfloatval_t *x, lbfgsfloatval_t *g, const int n, const lbfgsfloatval_t step) {
-    MinimizerData* data = reinterpret_cast<MinimizerData*>(instance);
-    ContextImpl& context = data->context;
-    ComputeContext& cc = data->cc;
-    vector<int>& drudeParticles = data->drudeParticles;
-    int numDrudeParticles = drudeParticles.size();
-
-    // Set the particle positions.
-    
-    cc.getPosq().download(cc.getPinnedBuffer());
-    if (cc.getUseDoublePrecision()) {
-        mm_double4* posq = (mm_double4*) cc.getPinnedBuffer();
-        for (int i = 0; i < numDrudeParticles; ++i) {
-            mm_double4& p = posq[drudeParticles[i]];
-            p.x = x[3*i];
-            p.y = x[3*i+1];
-            p.z = x[3*i+2];
-        }
-    }
-    else {
-        mm_float4* posq = (mm_float4*) cc.getPinnedBuffer();
-        for (int i = 0; i < numDrudeParticles; ++i) {
-            mm_float4& p = posq[drudeParticles[i]];
-            p.x = x[3*i];
-            p.y = x[3*i+1];
-            p.z = x[3*i+2];
-        }
-    }
-    cc.getPosq().upload(cc.getPinnedBuffer());
-
-    // Compute the forces and energy for this configuration.
-
-    double energy = context.calcForcesAndEnergy(true, true, context.getIntegrator().getIntegrationForceGroups());
-    long long* force = (long long*) cc.getPinnedBuffer();
-    cc.getLongForceBuffer().download(force);
-    double forceScale = -1.0/0x100000000;
-    int paddedNumAtoms = cc.getPaddedNumAtoms();
-    for (int i = 0; i < numDrudeParticles; ++i) {
-        int index = drudeParticles[i];
-        g[3*i] = forceScale*force[index];
-        g[3*i+1] = forceScale*force[index+paddedNumAtoms];
-        g[3*i+2] = forceScale*force[index+paddedNumAtoms*2];
-    }
-    return energy;
-}
-
 void CommonIntegrateDrudeSCFStepKernel::minimize(ContextImpl& context, double tolerance) {
-    // Record the initial positions.
-
-    int numDrudeParticles = drudeParticles.size();
-    cc.getPosq().download(cc.getPinnedBuffer());
-    if (cc.getUseDoublePrecision()) {
-        mm_double4* posq = (mm_double4*) cc.getPinnedBuffer();
-        for (int i = 0; i < numDrudeParticles; ++i) {
-            mm_double4 p = posq[drudeParticles[i]];
-            minimizerPos[3*i] = p.x;
-            minimizerPos[3*i+1] = p.y;
-            minimizerPos[3*i+2] = p.z;
-        }
-    }
-    else {
-        mm_float4* posq = (mm_float4*) cc.getPinnedBuffer();
-        for (int i = 0; i < numDrudeParticles; ++i) {
-            mm_float4 p = posq[drudeParticles[i]];
-            minimizerPos[3*i] = p.x;
-            minimizerPos[3*i+1] = p.y;
-            minimizerPos[3*i+2] = p.z;
+    minimizeKernel->setArg(2, (float) tolerance);
+    long long* forces = (long long*) cc.getPinnedBuffer();
+    double scale = 1/(double) 0x100000000;
+    double lastForce = 0;
+    int numDrude = drudeParams.getSize();
+    int paddedNumAtoms = cc.getPaddedNumAtoms();
+    for (int iteration = 0; iteration < 50; iteration++) {
+        context.calcForcesAndEnergy(true, false, context.getIntegrator().getIntegrationForceGroups());
+        minimizeKernel->execute(drudeParams.getSize());
+        cc.getLongForceBuffer().download(forces);
+        double totalForce = 0;
+        for (int i : drudeIndexVec) {
+            Vec3 f(scale*forces[i], scale*forces[i+paddedNumAtoms], scale*forces[i+paddedNumAtoms*2]);
+            totalForce += f.dot(f);
         }
-        minimizerParams.xtol = 1e-7;
+        if (sqrt(totalForce/(3*numDrude)) < tolerance || (iteration > 0 && totalForce > 0.9*lastForce)) 
+            break;
+        lastForce = totalForce;
     }
-    
-    // Determine a normalization constant for scaling the tolerance.
-    
-    double norm = 0.0;
-    for (int i = 0; i < 3*numDrudeParticles; i++)
-        norm += minimizerPos[i]*minimizerPos[i];
-    norm /= numDrudeParticles;
-    norm = (norm < 1 ? 1 : sqrt(norm));
-    minimizerParams.epsilon = tolerance/norm;
-    
-    // Perform the minimization.
-
-    lbfgsfloatval_t fx;
-    MinimizerData data(context, cc, drudeParticles);
-    lbfgs(numDrudeParticles*3, minimizerPos, &fx, evaluate, NULL, &data, &minimizerParams);
-}
+}
\ No newline at end of file

plugins/drude/platforms/common/src/CommonDrudeKernels.h

@@ -9,7 +9,7 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2013-2019 Stanford University and the Authors.      *
+ * Portions copyright (c) 2013-2023 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -35,7 +35,6 @@
 #include "openmm/DrudeKernels.h"
 #include "openmm/common/ComputeContext.h"
 #include "openmm/common/ComputeArray.h"
-#include "lbfgs.h"
 
 namespace OpenMM {
 
@@ -121,7 +120,7 @@ private:
 class CommonIntegrateDrudeSCFStepKernel : public IntegrateDrudeSCFStepKernel {
 public:
     CommonIntegrateDrudeSCFStepKernel(const std::string& name, const Platform& platform, ComputeContext& cc) :
-            IntegrateDrudeSCFStepKernel(name, platform), cc(cc), minimizerPos(NULL), hasInitializedKernels(false) {
+            IntegrateDrudeSCFStepKernel(name, platform), cc(cc), hasInitializedKernels(false) {
     }
     ~CommonIntegrateDrudeSCFStepKernel();
     /**
@@ -151,10 +150,9 @@ private:
     ComputeContext& cc;
     double prevStepSize;
     bool hasInitializedKernels;
-    std::vector<int> drudeParticles;
-    lbfgsfloatval_t *minimizerPos;
-    lbfgs_parameter_t minimizerParams;
-    ComputeKernel kernel1, kernel2;
+    std::vector<int> drudeIndexVec;
+    ComputeArray drudeParams, drudeIndices, drudeParents;
+    ComputeKernel kernel1, kernel2, minimizeKernel;
 };
 
 } // namespace OpenMM

plugins/drude/platforms/common/src/kernels/drudeSCF.cc

+KERNEL void minimizeDrudePositions(int numDrude, int paddedNumAtoms, float tolerance, GLOBAL real4* RESTRICT posq,
+        GLOBAL const mm_long* RESTRICT force, GLOBAL float4* RESTRICT drudeParams, GLOBAL int* RESTRICT drudeIndex,
+        GLOBAL int4* RESTRICT drudeParents) {
+    const real scale = 1/(real) 0x100000000;
+    for (int i = GLOBAL_ID; i < numDrude; i += GLOBAL_SIZE) {
+        int index = drudeIndex[i];
+        int4 parents = drudeParents[i];
+        float4 params = drudeParams[i];
+        real3 fscale = make_real3(params.z, params.z, params.z);
+        if (parents.y != -1) {
+            real3 dir = trimTo3(posq[parents.x]-posq[parents.y]);
+            dir *= RSQRT(dot(dir, dir));
+            fscale += params.x*dir;
+        }
+        if (parents.z != -1 && parents.w != -1) {
+            real3 dir = trimTo3(posq[parents.z]-posq[parents.w]);
+            dir *= RSQRT(dot(dir, dir));
+            fscale += params.y*dir;
+        }
+        real4 pos = posq[index];
+        real4 f = make_real4(scale*force[index], scale*force[index+paddedNumAtoms], scale*force[index+paddedNumAtoms*2], 0);
+        real damping = (SQRT(f.x*f.x + f.y*f.y + f.z*f.z) > 10*tolerance ? 0.5f : 1.0f);
+        pos.x += damping*f.x/fscale.x;
+        pos.y += damping*f.y/fscale.y;
+        pos.z += damping*f.z/fscale.z;
+        posq[index] = pos;
+    }
+}
+

plugins/drude/platforms/reference/src/ReferenceDrudeKernels.cpp

@@ -6,7 +6,7 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2011-2022 Stanford University and the Authors.      *
+ * Portions copyright (c) 2011-2023 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -384,18 +384,29 @@ double ReferenceIntegrateDrudeLangevinStepKernel::computeKineticEnergy(ContextIm
 }
 
 ReferenceIntegrateDrudeSCFStepKernel::~ReferenceIntegrateDrudeSCFStepKernel() {
-    if (minimizerPos != NULL)
-        lbfgs_free(minimizerPos);
 }
 
 void ReferenceIntegrateDrudeSCFStepKernel::initialize(const System& system, const DrudeSCFIntegrator& integrator, const DrudeForce& force) {
-    // Identify Drude particles.
-    
-    for (int i = 0; i < force.getNumParticles(); i++) {
-        int p, p1, p2, p3, p4;
+    // Record information about the drude particles.
+
+    int numDrude = force.getNumParticles();
+    particle.resize(numDrude);
+    particle1.resize(numDrude);
+    particle2.resize(numDrude);
+    particle3.resize(numDrude);
+    particle4.resize(numDrude);
+    k1.resize(numDrude);
+    k2.resize(numDrude);
+    k3.resize(numDrude);
+    for (int i = 0; i < numDrude; i++) {
         double charge, polarizability, aniso12, aniso34;
-        force.getParticleParameters(i, p, p1, p2, p3, p4, charge, polarizability, aniso12, aniso34);
-        drudeParticles.push_back(p);
+        force.getParticleParameters(i, particle[i], particle1[i], particle2[i], particle3[i], particle4[i], charge, polarizability, aniso12, aniso34);
+        double a1 = (particle2[i] == -1 ? 1 : aniso12);
+        double a2 = (particle3[i] == -1 || particle4[i] == -1 ? 1 : aniso34);
+        double a3 = 3-a1-a2;
+        k3[i] = ONE_4PI_EPS0*charge*charge/(polarizability*a3);
+        k1[i] = ONE_4PI_EPS0*charge*charge/(polarizability*a1) - k3[i];
+        k2[i] = ONE_4PI_EPS0*charge*charge/(polarizability*a2) - k3[i];
     }
 
     // Record particle masses.
@@ -406,16 +417,6 @@ void ReferenceIntegrateDrudeSCFStepKernel::initialize(const System& system, cons
         particleMass.push_back(mass);
         particleInvMass.push_back(mass == 0.0 ? 0.0 : 1.0/mass);
     }
-    
-    // Initialize the energy minimizer.
-    
-    minimizerPos = lbfgs_malloc(drudeParticles.size()*3);
-    if (minimizerPos == NULL)
-        throw OpenMMException("DrudeSCFIntegrator: Failed to allocate memory");
-    lbfgs_parameter_init(&minimizerParams);
-    minimizerParams.linesearch = LBFGS_LINESEARCH_BACKTRACKING_STRONG_WOLFE;
-    if (sizeof(double) < 8)
-        minimizerParams.xtol = 1e-7;
 }
 
 void ReferenceIntegrateDrudeSCFStepKernel::execute(ContextImpl& context, const DrudeSCFIntegrator& integrator) {
@@ -461,54 +462,40 @@ double ReferenceIntegrateDrudeSCFStepKernel::computeKineticEnergy(ContextImpl& c
     return computeShiftedKineticEnergy(context, particleInvMass, 0.5*integrator.getStepSize());
 }
 
-struct MinimizerData {
-    ContextImpl& context;
-    vector<int>& drudeParticles;
-    MinimizerData(ContextImpl& context, vector<int>& drudeParticles) : context(context), drudeParticles(drudeParticles) {}
-};
-
-static lbfgsfloatval_t evaluate(void *instance, const lbfgsfloatval_t *x, lbfgsfloatval_t *g, const int n, const lbfgsfloatval_t step) {
-    MinimizerData* data = reinterpret_cast<MinimizerData*>(instance);
-    ContextImpl& context = data->context;
-    vector<int>& drudeParticles = data->drudeParticles;
-    int numDrudeParticles = drudeParticles.size();
-
-    // Compute the force and energy for this configuration.
-
-    vector<Vec3>& pos = extractPositions(context);
-    vector<Vec3>& force = extractForces(context);
-    for (int i = 0; i < numDrudeParticles; i++)
-        pos[drudeParticles[i]] = Vec3(x[3*i], x[3*i+1], x[3*i+2]);
-    double energy = context.calcForcesAndEnergy(true, true, context.getIntegrator().getIntegrationForceGroups());
-    for (int i = 0; i < numDrudeParticles; i++) {
-        Vec3 f = force[drudeParticles[i]];
-        g[3*i] = -f[0];
-        g[3*i+1] = -f[1];
-        g[3*i+2] = -f[2];
-    }
-    return energy;
-}
-
 void ReferenceIntegrateDrudeSCFStepKernel::minimize(ContextImpl& context, double tolerance) {
-    // Record the initial positions and determine a normalization constant for scaling the tolerance.
-
     vector<Vec3>& pos = extractPositions(context);
-    int numDrudeParticles = drudeParticles.size();
-    double norm = 0.0;
-    for (int i = 0; i < numDrudeParticles; i++) {
-        Vec3 p = pos[drudeParticles[i]];
-        minimizerPos[3*i] = p[0];
-        minimizerPos[3*i+1] = p[1];
-        minimizerPos[3*i+2] = p[2];
-        norm += p.dot(p);
+    vector<Vec3>& force = extractForces(context);
+    int numDrude = particle.size();
+    double lastForce = 0;
+    for (int iteration = 0; iteration < 50; iteration++) {
+        context.calcForcesAndEnergy(true, false, context.getIntegrator().getIntegrationForceGroups());
+        double totalForce = 0;
+        for (int i = 0; i < numDrude; i++) {
+            int p = particle[i];
+            int p1 = particle1[i];
+            int p2 = particle2[i];
+            int p3 = particle3[i];
+            int p4 = particle4[i];
+            Vec3 fscale(k3[i], k3[i], k3[i]);
+            if (p2 != -1) {
+                Vec3 dir = pos[p1]-pos[p2];
+                dir /= sqrt(dir.dot(dir));;
+                fscale += k1[i]*dir;
+            }
+            if (p3 != -1 && p4 != -1) {
+                Vec3 dir = pos[p3]-pos[p4];
+                dir /= sqrt(dir.dot(dir));;
+                fscale += k2[i]*dir;
+            }
+            Vec3 f = force[p];
+            double f2 = f.dot(f);
+            totalForce += f2;
+            double damping = (sqrt(f2) > 10*tolerance ? 0.5 : 1.0);
+            for (int i = 0; i < 3; i++)
+                pos[p][i] += damping*f[i]/fscale[i];
+        }
+        if (sqrt(totalForce/(3*numDrude)) < tolerance || (iteration > 0 && totalForce > 0.9*lastForce))
+            break;
+        lastForce = totalForce;
     }
-    norm /= numDrudeParticles;
-    norm = (norm < 1 ? 1 : sqrt(norm));
-    minimizerParams.epsilon = tolerance/norm;
-    
-    // Perform the minimization.
-
-    lbfgsfloatval_t fx;
-    MinimizerData data(context, drudeParticles);
-    lbfgs(numDrudeParticles*3, minimizerPos, &fx, evaluate, NULL, &data, &minimizerParams);
 }

plugins/drude/platforms/reference/src/ReferenceDrudeKernels.h

@@ -9,7 +9,7 @@
  * Biological Structures at Stanford, funded under the NIH Roadmap for        *
  * Medical Research, grant U54 GM072970. See https://simtk.org.               *
  *                                                                            *
- * Portions copyright (c) 2013 Stanford University and the Authors.           *
+ * Portions copyright (c) 2013-2023 Stanford University and the Authors.      *
  * Authors: Peter Eastman                                                     *
  * Contributors:                                                              *
  *                                                                            *
@@ -35,7 +35,6 @@
 #include "ReferencePlatform.h"
 #include "openmm/DrudeKernels.h"
 #include "openmm/Vec3.h"
-#include "lbfgs.h"
 #include <utility>
 #include <vector>
 
@@ -127,7 +126,7 @@ private:
 class ReferenceIntegrateDrudeSCFStepKernel : public IntegrateDrudeSCFStepKernel {
 public:
     ReferenceIntegrateDrudeSCFStepKernel(const std::string& name, const Platform& platform, ReferencePlatform::PlatformData& data) :
-        IntegrateDrudeSCFStepKernel(name, platform), data(data), minimizerPos(NULL) {
+        IntegrateDrudeSCFStepKernel(name, platform), data(data) {
     }
     ~ReferenceIntegrateDrudeSCFStepKernel();
     /**
@@ -155,11 +154,10 @@ public:
 private:
     void minimize(ContextImpl& context, double tolerance);
     ReferencePlatform::PlatformData& data;
-    std::vector<int> drudeParticles;
     std::vector<double> particleInvMass;
-    lbfgsfloatval_t *minimizerPos;
-    lbfgs_parameter_t minimizerParams;
     double maxDrudeDistance;
+    std::vector<int> particle, particle1, particle2, particle3, particle4;
+    std::vector<double> k1, k2, k3;
 };
 
 } // namespace OpenMM

plugins/drude/tests/TestDrudeSCFIntegrator.h

@@ -100,6 +100,7 @@ void testWater() {
     // Simulate it and check energy conservation and the total force on the Drude particles.
 
     DrudeSCFIntegrator integ(0.0005);
+    integ.setMinimizationErrorTolerance(0.1);
     Context context(system, integ, platform);
     context.setPositions(positions);
     context.applyConstraints(1e-5);

wrappers/python/tests/TestAPIUnits.py

@@ -1050,10 +1050,10 @@ class TestAPIUnits(unittest.TestCase):
         integrator = DrudeSCFIntegrator(0.002)
         self.assertEqual(integrator.getStepSize(), 0.002*picoseconds)
         self.assertAlmostEqualUnit(integrator.getMinimizationErrorTolerance(),
-                                   0.1*kilojoule_per_mole/nanometer)
-        integrator.setMinimizationErrorTolerance(1*kilocalorie_per_mole/angstrom)
+                                   1*kilojoule_per_mole/nanometer)
+        integrator.setMinimizationErrorTolerance(0.1*kilocalorie_per_mole/angstrom)
         self.assertAlmostEqualUnit(integrator.getMinimizationErrorTolerance(),
-                                   1*kilocalorie_per_mole/angstrom)
+                                   0.1*kilocalorie_per_mole/angstrom)
 
     def testDrudeLangevinIntegrator(self):
         """ Tests the DrudeLangevinIntegrator API features """