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force.cpp
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Thu, Feb 20, 06:45

force.cpp
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/*
//@HEADER
// ************************************************************************
//
// Kokkos v. 2.0
// Copyright (2014) Sandia Corporation
//
// Under the terms of Contract DE-AC04-94AL85000 with Sandia Corporation,
// the U.S. Government retains certain rights in this software.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// 1. Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//
// 2. Redistributions in binary form must reproduce the above copyright
// notice, this list of conditions and the following disclaimer in the
// documentation and/or other materials provided with the distribution.
//
// 3. Neither the name of the Corporation nor the names of the
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY SANDIA CORPORATION "AS IS" AND ANY
// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
// PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL SANDIA CORPORATION OR THE
// CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
// EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
// Questions? Contact H. Carter Edwards (hcedwar@sandia.gov)
//
// ************************************************************************
//@HEADER
*/
/* Define values which set the max number of registers used for the Force Kernel
* Its 32 * 2048 / (KOKKOS_CUDA_MAX_THREADS * KOKKOS_CUDA_MIN_BLOCKS)
* Have to be set before including Kokkos header files.
*/
#define KOKKOS_CUDA_MAX_THREADS 512
#define KOKKOS_CUDA_MIN_BLOCKS 3
#include <system.h>
#include <cstdio>
/* Simple Lennard Jones Force Kernel using neighborlists
* Calculates for every pair of atoms (i,j) with distance smaller r_cut
* f_ij = 4*epsilon * ( (sigma/r_ij)^12 - (sigma/r_ij)^6 )
* where r_ij is the distance of atoms (i,j).
* The force on atom i is the sum over f_ij:
* f_i = sum_j (f_ij)
* Neighborlists are used in order to pre calculate which atoms j are
* close enough to i to be able to contribute. By choosing a larger neighbor
* cutoff then the force cutoff, the neighbor list can be reused several times
* (typically 10 - 100).
*/
struct ForceFunctor {
typedef t_x_array::execution_space execution_space; //Device Type for running the kernel
typedef double2 value_type; // When energy calculation is requested return energy, and virial
t_x_array_randomread x; //atom positions
t_f_array f; //atom forces
t_int_1d_const numneigh; //number of neighbors per atom
t_neighbors_const neighbors; //neighborlist
double cutforcesq; //force cutoff
double epsilon; //Potential parameter
double sigma6; //Potential parameter
ForceFunctor(System s) {
x = s.d_x;
f = s.f;
numneigh = s.numneigh;
neighbors = s.neighbors;
cutforcesq = s.force_cutsq;
epsilon = 1.0;
sigma6 = 1.0;
}
/* Operator for not calculating energy and virial */
KOKKOS_INLINE_FUNCTION
void operator() (const int &i) const {
force<0>(i);
}
/* Operator for calculating energy and virial */
KOKKOS_INLINE_FUNCTION
void operator() (const int &i, double2 &energy_virial) const {
double2 ev = force<1>(i);
energy_virial.x += ev.x;
energy_virial.y += ev.y;
}
template<int EVFLAG>
KOKKOS_INLINE_FUNCTION
double2 force(const int &i) const
{
const int numneighs = numneigh[i];
const double xtmp = x(i, 0);
const double ytmp = x(i, 1);
const double ztmp = x(i, 2);
double fix = 0;
double fiy = 0;
double fiz = 0;
double energy = 0;
double virial = 0;
//pragma simd forces vectorization (ignoring the performance objections of the compiler)
//give hint to compiler that fix, fiy and fiz are used for reduction only
#ifdef USE_SIMD
#pragma simd reduction (+: fix,fiy,fiz,energy,virial)
#endif
for(int k = 0; k < numneighs; k++) {
const int j = neighbors(i, k);
const double delx = xtmp - x(j, 0);
const double dely = ytmp - x(j, 1);
const double delz = ztmp - x(j, 2);
const double rsq = delx * delx + dely * dely + delz * delz;
//if(i==0) printf("%i %i %lf %lf\n",i,j,rsq,cutforcesq);
if(rsq < cutforcesq) {
const double sr2 = 1.0 / rsq;
const double sr6 = sr2 * sr2 * sr2 * sigma6;
const double force = 48.0 * sr6 * (sr6 - 0.5) * sr2 * epsilon;
fix += delx * force;
fiy += dely * force;
fiz += delz * force;
if(EVFLAG) {
energy += sr6 * (sr6 - 1.0) * epsilon;
virial += delx * delx * force + dely * dely * force + delz * delz * force;
}
}
}
f(i, 0) += fix;
f(i, 1) += fiy;
f(i, 2) += fiz;
double2 energy_virial ;
energy_virial.x = 4.0 * energy ;
energy_virial.y = 0.5 * virial ;
return energy_virial;
}
/* init and join functions when doing the reduction to obtain energy and virial */
KOKKOS_FUNCTION
static void init(volatile value_type &update) {
update.x = update.y = 0;
}
KOKKOS_FUNCTION
static void join(volatile value_type &update ,
const volatile value_type &source) {
update.x += source.x ;
update.y += source.y ;
}
};
/* Calling function */
double2 force(System &s,int evflag) {
ForceFunctor f(s);
double2 ev ; ev.x = 0 ; ev.y = 0 ;
if(!evflag)
Kokkos::parallel_for(s.nlocal,f);
else
Kokkos::parallel_reduce(s.nlocal,f,ev);
execution_space::fence();
return ev;
}

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