LHDForceModifier.hpp

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/*! \file integrators/LHDForceModifier.hpp
    \brief Langevin force modifier with intramolecular RPY hydrodynamic
           interactions for flexible bead molecules in a background flow.
 
    Translation-only (unstructured spherical atom) case. Each molecule is a
    hydrodynamically coupled group of N spherical beads; molecules do not
    interact hydrodynamically with each other. Per step and per molecule:
 
      - assemble the 3N x 3N RPY mobility and its resistance R = M^{-1}
        (RPYMobility), Cholesky factor S of R;
      - sample the background flow at each bead (VelocityField) and fold in
        the dipolar disturbance to get v_inf_eff;
      - add the FDT-consistent correlated random force sqrt(2 kT/dt) S Z;
      - solve the coupled friction f = R (v_inf_eff - v) self-consistently for
        the full-step velocity (the LDForceModifier iteration, now coupled
        across the molecule's beads).
 
    Pairs with a velocity-Verlet (LangevinDynamics-style) integrator, which
    performs the actual velocity update; this modifier only injects forces.
*/
 
#ifndef INTEGRATOR_LHDFORCEMODIFIER_HPP
#define INTEGRATOR_LHDFORCEMODIFIER_HPP
 
#include <memory>
#include <random>
#include <vector>
 
#include "brains/ForceModifier.hpp"
#include "brains/Velocitizer.hpp"
#include "hydrodynamics/RPYMobility.hpp"
#include "perturbations/VelocityField.hpp"
#include "primitives/Atom.hpp"
#include "primitives/StuntDouble.hpp"
#include "utils/RandNumGen.hpp"
 
namespace OpenMD {
 
  class LHDForceModifier : public ForceModifier {
  public:
    LHDForceModifier(SimInfo* info);
 
    void modifyForces() override;
 
    int getMaxIterationNumber() const { return maxIterNum_; }
    void setMaxIterationNumber(int n) { maxIterNum_ = n; }
    RealType getForceTolerance() const { return forceTolerance_; }
    void setForceTolerance(RealType t) { forceTolerance_ = t; }
 
  private:
    //! Hydrodynamic radius of a bead, following the LDForceModifier rules:
    //! Lennard-Jones sigma/2, else the element van der Waals radius.
    RealType beadRadius(Atom* atom) const;
 
    //! One hydrodynamically coupled group (a molecule).
    struct HydroMolecule {
      std::vector<StuntDouble*> beads;
      std::vector<RealType> masses;
      std::unique_ptr<RPYMobility> mobility;
    };
 
    std::vector<HydroMolecule> molecules_;
 
    int maxIterNum_;
    RealType forceTolerance_;
    RealType dt_;
    RealType dt2_;
    RealType viscosity_;
    RealType kT_;  // Constants::kb * targetTemp
 
    Globals* simParams_;
    Utils::RandNumGenPtr randNumGen_;
    std::normal_distribution<RealType> normal_ {0.0, 1.0};
    std::unique_ptr<VelocityField> velField_;
    std::unique_ptr<Velocitizer> veloMunge_;
  };
}  // namespace OpenMD
 
#endif  // INTEGRATOR_LHDFORCEMODIFIER_HPP