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CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

Building

All build artifacts go in build/. The source is in src/.

cd build
cmake ../src -DLATTICE=chain        # required: chain | square | cubic | ladder | triangular | honeycomb
make -j4

Key CMake flags (all optional):

Flag Default Effect
-DLATTICE=<name> chain Lattice geometry — must match the physics you want
-DDSTRUC=AVL|LIST|LIST_STACK LIST Internal data structure for the diagram
-DDEBUG=ON OFF Enables DEBUGMODE: runs test_conf() after every update
-DUNIFORM=OFF ON Uniform (UNISYS) vs site/bond-inhomogeneous parameters
-DCWINDOW=ON OFF Canonical window for Green's function measurement

This produces two executables: qmc_worm (single-core) and qmc_worm_mpi (MPI).

Running

./build/qmc_worm parameter_files/BoseHubbard.ini
mpiexec -n 4 ./build/qmc_worm_mpi parameter_files/BoseHubbard.ini

Resume from checkpoint: ./qmc_worm job.clone.h5

Output is HDF5 (job.out.h5). Helper scripts in tools/ extract results:

python tools/print_params.py job.out.h5
python tools/print_corrfun.py job.out.h5
python tools/print_update_statistics.py job.out.h5

Tests

Benchmarks against exact Lanczos diagonalization live in test_mpi/:

cd test_mpi/BoseHubbard_GrandCanonical
bash test_serial.sh chain        # runs cmake + make + simulation, then compare.py checks results

Subdirectories: BoseHubbard_GrandCanonical, BoseHubbard_Canonical, XXZ_SpinOneHalf, XXZ_SpinOne.

Architecture

Entry points

  • src/worm.run.cpp — single-core main()
  • src/worm.run_mpi.cpp — MPI main()

Both construct a worm object and hand off to the ALPSCore MC driver loop.

Core class: worm (src/worm.hpp, src/worm.cpp)

Inherits alps::mcbase. Owns the diagram (a Diagram_type container of Elements per site), the worm head/tail iterators, and all measurement accumulators. Key members:

  • worm_diag — bool distinguishing the diagonal (no worm) from off-diagonal (worm present) sector
  • update_prob_cuml[] — cumulative probabilities for selecting each update

MC updates (src/worm.update.cpp)

Five updates: INSERTWORM, GLUEWORM, MOVEWORM, INSERTKINK, DELETEKINK. Called from worm::update() which dispatches based on worm_diag.

Data structure (src/worm.Element.hpp)

Element stores a single vertex: imaginary time, site index (mLink), occupation before/after (mBefore/mAfter), color (boson kink = +1, worm = −1, dummy = 0), and mAssoc[ZC] — iterators to the nearest neighbor elements just ahead in time on each neighboring site. Three interchangeable containers selectable at compile time via -DDSTRUC: AVL_diagram, list_diagram, list_stack_diagram.

The operator< tie-breaking for equal-time elements (comparing after() vs before()) is an intentional physics-based workaround: two worm elements at the same time on the same site form a chain via continuous occupancy, making the ordering unambiguous. Do not treat this as a bug.

Models (src/model.hpp)

Abstract base model with two concrete implementations: BoseHubbard and XXZ. Each provides diagonal/off-diagonal site and bond weights. Compiled with UNISYS (uniform parameters, scalars) or without (inhomogeneous, vectors indexed by site/bond).

Lattice (src/lattice.hpp)

Template lattice<DIM, N_BASIS> selected at compile time via -DLATTICE=. Provides neighbor maps, bond lists, and boundary condition logic. LATTICE::zcmax sets the compile-time maximum coordination number used to size mAssoc[ZC] in every Element.

src_xml/

Older variant of the source — kept for reference but not the active build target.