libhoomd/computes/EvaluatorConstraint.h

   1 /*
   2 Highly Optimized Object-oriented Many-particle Dynamics -- Blue Edition
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  49
  50 // Maintainer: joaander
  51
  52 #ifndef __EVALUATOR_CONSTRAINT_H__
  53 #define __EVALUATOR_CONSTRAINT_H__
  54
  55 #include "HOOMDMath.h"
  56
  57 /*! \file EvaluatorConstraint.h
  58     \brief Defines basic evaluation methods common to all constraint force implementations
  59 */
  60
  61 // need to declare these class methods with __device__ qualifiers when building in nvcc
  62 // DEVICE is __host__ __device__ when included in nvcc and blank when included into the host compiler
  63 #ifdef NVCC
  64 #define DEVICE __device__
  65 #else
  66 #define DEVICE
  67 #endif
  68
  69 //! Class for evaluating constraint forces
  70 /*! <b>General Overview</b>
  71     EvaluatorConstraint is a low level computation class for use on the CPU and GPU. It provides basic functionality
  72     needed by all constraint forces.
  73 */
  74 class EvaluatorConstraint
  75     {
  76     public:
  77         //! Constructs the constraint evaluator
  78         /*! \param _X Current position at the time of the constraint force calculation
  79             \param V Current velocity at the time of the constraint force calculation
  80             \param F Current net force at the time of the constraint force calculation
  81             \param _m Mass of the particle
  82             \param _deltaT Step size delta t
  83         */
  84         DEVICE EvaluatorConstraint(Scalar3 _X, Scalar3 V, Scalar3 F, Scalar _m, Scalar _deltaT)
  85             : X(_X), m(_m), deltaT(_deltaT)
  86             {
  87             // perform step 2 of this velocity verlet update and step 1 of the next to get
  88             // U = X(t+2deltaT) given X = X(t+deltaT)
  89             Scalar minv = Scalar(1.0)/m;
  90             Scalar dtsqdivm = deltaT*deltaT * minv;
  91             U.x = X.x + V.x * deltaT + F.x * dtsqdivm;
  92             U.y = X.y + V.y * deltaT + F.y * dtsqdivm;
  93             U.z = X.z + V.z * deltaT + F.z * dtsqdivm;
  94             }
  95
  96         //! Evaluate the unconstrained position update U
  97         /*! \returns The unconstrained position update U
  98         */
  99         DEVICE Scalar3 evalU()
 100             {
 101             return U;
 102             }
 103
 104         //! Evaluate the additional constraint force
 105         /*! \param FC output parameter where the computed force is written
 106             \param virial array of six scalars the computed virial tensor is written
 107             \param C constrained position particle will be moved to at the next step
 108             \return Additional force \a F needed to satisfy the constraint
 109         */
 110         DEVICE void evalConstraintForce(Scalar3& FC, Scalar *virial, const Scalar3& C)
 111             {
 112             // subtract a constrained update from U and get that F = (C-U)*m/dt^2
 113             Scalar moverdtsq = m / (deltaT * deltaT);
 114             FC.x = (C.x - U.x) * moverdtsq;
 115             FC.y = (C.y - U.y) * moverdtsq;
 116             FC.z = (C.z - U.z) * moverdtsq;
 117
 118             // compute virial
 119             virial[0] = FC.x * X.x;
 120             virial[1] = Scalar(1./2.)*(FC.y * X.x + FC.x * X.y);
 121             virial[2] = Scalar(1./2.)*(FC.z * X.x + FC.x * X.z);
 122             virial[3] = FC.y * X.y;
 123             virial[4] = Scalar(1./2.)*(FC.z * X.y + FC.y * X.z);
 124             virial[5] = FC.z * X.z;
 125             }
 126
 127     protected:
 128         Scalar3 U;      //!< Unconstrained position update
 129         Scalar3 X;      //!< Current particle position
 130         Scalar m;       //!< Saved mass value
 131         Scalar deltaT;  //!< Saved delta T value
 132
 133     };
 134
 135
 136 #endif // __PAIR_EVALUATOR_LJ_H__