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Merged
pull merged 2 commits into from
Aug 2, 2026
Merged

[pull] develop from lammps:develop #179

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03ea88c
Fix harmonic force for fix spring/chunk
csmonago Aug 1, 2026
8721bc3
Merge pull request #5120 from csmonago/fix-spring-chunk-force
akohlmey Aug 1, 2026
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16 changes: 6 additions & 10 deletions src/fix_spring_chunk.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -24,14 +24,11 @@
#include "respa.h"
#include "update.h"

#include <cmath>
#include <cstring>

using namespace LAMMPS_NS;
using namespace FixConst;

static constexpr double SMALL = 1.0e-10;

/* ---------------------------------------------------------------------- */

FixSpringChunk::FixSpringChunk(LAMMPS *lmp, int narg, char **arg) :
Expand Down Expand Up @@ -138,7 +135,7 @@ void FixSpringChunk::min_setup(int vflag)
void FixSpringChunk::post_force(int /*vflag*/)
{
int i,m;
double dx,dy,dz,r;
double dx,dy,dz,rsq;

// check if first time cchunk will be queried via ccom
// if so, lock idchunk for as long as this fix is in place
Expand Down Expand Up @@ -178,14 +175,13 @@ void FixSpringChunk::post_force(int /*vflag*/)
dx = com[m][0] - com0[m][0];
dy = com[m][1] - com0[m][1];
dz = com[m][2] - com0[m][2];
r = sqrt(dx*dx + dy*dy + dz*dz);
r = MAX(r,SMALL);
rsq = dx*dx + dy*dy + dz*dz;

if (masstotal[m] != 0.0) {
fcom[m][0] = k_spring*dx/r / masstotal[m];
fcom[m][1] = k_spring*dy/r / masstotal[m];
fcom[m][2] = k_spring*dz/r / masstotal[m];
esprings += 0.5*k_spring*r*r;
fcom[m][0] = k_spring*dx / masstotal[m];
fcom[m][1] = k_spring*dy / masstotal[m];
fcom[m][2] = k_spring*dz / masstotal[m];
esprings += 0.5*k_spring*rsq;
}
}

Expand Down
4 changes: 4 additions & 0 deletions unittest/commands/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,10 @@ add_executable(test_lattice_region test_lattice_region.cpp)
target_link_libraries(test_lattice_region PRIVATE lammps GTest::GMock)
add_test(NAME LatticeRegion COMMAND test_lattice_region)

add_executable(test_fix_spring_chunk test_fix_spring_chunk.cpp)
target_link_libraries(test_fix_spring_chunk PRIVATE lammps GTest::GMock)
add_test(NAME FixSpringChunk COMMAND test_fix_spring_chunk)

add_executable(test_kspace_auto_slab test_kspace_auto_slab.cpp)
target_compile_definitions(test_kspace_auto_slab PRIVATE -DTEST_INPUT_FOLDER=${CMAKE_CURRENT_SOURCE_DIR})
target_link_libraries(test_kspace_auto_slab PRIVATE lammps GTest::GMock)
Expand Down
136 changes: 136 additions & 0 deletions unittest/commands/test_fix_spring_chunk.cpp
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Original file line number Diff line number Diff line change
@@ -0,0 +1,136 @@
/* ----------------------------------------------------------------------
LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
https://www.lammps.org/, Sandia National Laboratories
LAMMPS Development team: developers@lammps.org

Copyright (2003) Sandia Corporation. Under the terms of Contract
DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
certain rights in this software.

See the README file in the top-level LAMMPS directory.
------------------------------------------------------------------------- */

#include "../testing/core.h"

#include "atom.h"
#include "fix.h"
#include "modify.h"
#include "utils.h"

#include "gmock/gmock.h"
#include "gtest/gtest.h"

#include <cstdlib>
#include <cstring>
#include <mpi.h>
#include <vector>

// whether to print verbose output (i.e. not capturing LAMMPS screen output).
bool verbose = false;

namespace LAMMPS_NS {

class FixSpringChunkTest : public LAMMPSTest {
protected:
void SetUp() override
{
testbinary = "FixSpringChunkTest";
LAMMPSTest::SetUp();

BEGIN_HIDE_OUTPUT();
command("units lj");
command("atom_style atomic");
command("boundary f f f");
command("region box block -2 2 -2 2 -2 2");
command("create_box 1 box");
command("create_atoms 1 single 0.0 0.0 0.0 units box");
command("mass 1 1.0");
command("pair_style zero 1.0");
command("pair_coeff * *");
command("compute chunks all chunk/atom type");
command("compute centers all com/chunk chunks");
command("fix integrate all nve");
command("fix spring all spring/chunk 2.0 chunks centers");
command("run 0 post no");
END_HIDE_OUTPUT();
}
};

TEST_F(FixSpringChunkTest, HarmonicForceAndEnergy)
{
ASSERT_EQ(lmp->atom->nlocal, 1);

auto *spring = lmp->modify->get_fix_by_id("spring");
ASSERT_NE(spring, nullptr);

auto **force = lmp->atom->f;
EXPECT_DOUBLE_EQ(spring->compute_scalar(), 0.0);
EXPECT_DOUBLE_EQ(force[0][0], 0.0);
EXPECT_DOUBLE_EQ(force[0][1], 0.0);
EXPECT_DOUBLE_EQ(force[0][2], 0.0);

BEGIN_HIDE_OUTPUT();
command("displace_atoms all move 0.25 0.0 0.0 units box");
command("run 0 post no");
END_HIDE_OUTPUT();

EXPECT_DOUBLE_EQ(spring->compute_scalar(), 0.0625);
EXPECT_DOUBLE_EQ(force[0][0], -0.5);
EXPECT_DOUBLE_EQ(force[0][1], 0.0);
EXPECT_DOUBLE_EQ(force[0][2], 0.0);

// Independently verify that the applied force is the negative numerical
// derivative of the energy returned by the fix.

constexpr double delta = 1.0e-4;

BEGIN_HIDE_OUTPUT();
command("displace_atoms all move 0.0001 0.0 0.0 units box");
command("run 0 post no");
END_HIDE_OUTPUT();
const double eplus = spring->compute_scalar();

BEGIN_HIDE_OUTPUT();
command("displace_atoms all move -0.0002 0.0 0.0 units box");
command("run 0 post no");
END_HIDE_OUTPUT();
const double eminus = spring->compute_scalar();

BEGIN_HIDE_OUTPUT();
command("displace_atoms all move 0.0001 0.0 0.0 units box");
command("run 0 post no");
END_HIDE_OUTPUT();

const double numerical_force = -(eplus-eminus) / (2.0*delta);
EXPECT_NEAR(force[0][0], numerical_force, 1.0e-12);

BEGIN_HIDE_OUTPUT();
command("displace_atoms all move 0.25 0.0 0.0 units box");
command("run 0 post no");
END_HIDE_OUTPUT();

EXPECT_DOUBLE_EQ(spring->compute_scalar(), 0.25);
EXPECT_DOUBLE_EQ(force[0][0], -1.0);
}

} // namespace LAMMPS_NS

int main(int argc, char **argv)
{
MPI_Init(&argc, &argv);
::testing::InitGoogleMock(&argc, argv);

// handle arguments passed via environment variable
if (const char *var = getenv("TEST_ARGS")) {
std::vector<std::string> env = LAMMPS_NS::utils::split_words(var);
for (auto arg : env) {
if (arg == "-v") verbose = true;
}
}

if ((argc > 1) && (strcmp(argv[1], "-v") == 0)) verbose = true;

int rv = RUN_ALL_TESTS();
MPI_Finalize();
return rv;
}
132 changes: 66 additions & 66 deletions unittest/force-styles/tests/fix-timestep-spring_chunk.yaml
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Original file line number Diff line number Diff line change
@@ -1,79 +1,79 @@
---
lammps_version: 17 Feb 2022
date_generated: Fri Mar 18 22:18:01 2022
lammps_version: 4 Jul 2026
date_generated: Sat Aug 1 13:58:57 2026
epsilon: 1e-11
skip_tests:
prerequisites: ! |
prerequisites: |
atom full
fix spring/chunk
pre_commands: ! ""
post_commands: ! |
pre_commands: ""
post_commands: |
compute mol all chunk/atom molecule
compute com all com/chunk mol
fix move all nve
fix test solute spring/chunk 10.0 mol com
input_file: in.fourmol
input_coeffs: coeffs.fourmol
natoms: 29
global_scalar: 0.0015064306233739035
run_pos: ! |2
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run_vel: ! |2
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global_scalar: 0.001556817966196982
run_pos: |2
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...