ReferenceRGForce.cpp

/* Portions copyright (c) 2025 Stanford University and Simbios.
 * Contributors: Peter Eastman
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#include "ReferenceRGForce.h"
#include <cmath>

using namespace OpenMM;
using namespace std;

ReferenceRGForce::ReferenceRGForce(vector<int>& particles) : particles(particles) {
}

double ReferenceRGForce::calculateIxn(vector<Vec3>& atomCoordinates, vector<Vec3>& forces) const {
    // Compute the center position.

    int numParticles = particles.size();
    Vec3 center;
    for (int i : particles)
        center += atomCoordinates[i];
    center /= numParticles;

    // Compute the radius of gyration.

    double sum = 0.0;
    for (int i = 0; i < numParticles; i++) {
        Vec3 delta = atomCoordinates[particles[i]]-center;
        sum += delta.dot(delta);
    }
    double rg = sqrt(sum/numParticles);

    // Compute the forces.

    double scale = 1.0/(rg*numParticles);
    for (int i = 0; i < numParticles; i++) {
        Vec3 delta = atomCoordinates[particles[i]]-center;
        forces[particles[i]] -= scale*delta;
    }
    return rg;
}