QAssemble

QAssemble is a pure-Python quantum simulation package for calculating electronic properties of materials using free energy functional approaches. Built entirely with the Python standard library and a minimal set of well-established scientific packages — no compiled extensions or domain-specific frameworks required.

Why Pure Python?

QAssemble is intentionally implemented in pure Python, meaning:

• No C/C++/Fortran extensions beyond what NumPy/SciPy already provide
• No proprietary or hard-to-install domain-specific libraries
• Readable, hackable source code — every algorithm is visible and modifiable
• Easy to install, easy to extend, and easy to understand

Features

• Methods:
  • Tight-Binding (TB) — non-interacting band structure
  • Hartree-Fock (HF) — mean-field theory (restricted/unrestricted)
  • GW Approximation (GW) — many-body perturbation theory
• Advanced Numerics:
  • Discrete Lehmann Representation (DLR) for high-precision imaginary-time / Matsubara frequency transforms
  • Dyson equation solver for renormalized Green's functions
  • k-space / real-space Fourier transforms with phase-correct basis handling
  • High-frequency tail fitting for asymptotic accuracy
• Coulomb Interactions:
  • Local: Slater-Kanamori, Slater, Kanamori parameterizations
  • Non-local: Ohno, Ohno-Yukawa, J-threading (JTH)
• Input/Output: .ini-based configuration and HDF5 data storage via h5py
• Crystal Structure: Lattice vectors, basis positions, k-point grids, spin-orbit coupling (SOC)

Dependencies

Package	Purpose
NumPy	Array operations and linear algebra
SciPy	Eigensolvers, interpolation, special functions
h5py	HDF5-based data storage
Matplotlib	Plotting
pydlr	Discrete Lehmann Representation
SymPy	Wigner 3j symbols and Gaunt coefficients
pymatgen	Crystal structure utilities

Installation

From PyPI (recommended)

pip install QAssemble

From source (for development)

git clone https://github.com/QAssemble/qassemble.git
cd qassemble
pip install -e ".[test]"

After installation, the qassemble command will be available in your terminal.

Public Class API

The public many-body class hierarchy follows the names used in the QAssemble manuscript:

Base class	Physical classes
FLatDyn	G0, G, SigGWC
FLatStc	H0, H, SigH, SigF, Z, SigStc
BLatDyn	P, W
BLatStc	V

The foundational classes are CorrelationFunction, Crystal, and DLR. All of these names can be imported directly from QAssemble.

Migrating from 0.1

Version 0.2 adopts the manuscript names as the only supported class names:

QAssemble 0.1	QAssemble 0.2
NIHamiltonian	H0
Hamiltonian	H
SigmaHartree	SigH
SigmaFock	SigF
GreenBare	G0
GreenInt	G
SigmaGWC	SigGWC
PolLat	P
WLat	W
VBare	V

Existing result files must be migrated before restart or post-processing:

qassemble-migrate-hdf5 --dry-run result.h5
qassemble-migrate-hdf5 result.h5

The migration command creates result.h5.pre-class-rename.bak before replacing the original file. The input configuration section named Hamiltonian is not renamed.

Usage

1. Prepare Input

Create a qassemble.in file in your working directory. It defines the crystal structure, Hamiltonian parameters, and run settings as a restricted declarative QAssemble input file:

{
    "Crystal": {
        "RVec": [[1, 0, 0], [0.5, 0.866, 0], [0, 0, 1]],
        "SOC": False,
        "CorF": "F",
        "Basis": [
            [[0.33333, 0.33333, 0], 1],
            [[0.66667, 0.66667, 0], 1],
        ],
        "NSpin": 1,
        "NElec": 2,
        "KGrid": [25, 25, 1],
    },
    "Hamiltonian": {
        "OneBody": {
            "Hopping": {
                ((0, 0), (1, 0)): {
                    1.0: [[0, 0, 0], [-1, 0, 0], [0, -1, 0]],
                },
            },
            "Onsite": {
                0: {(0, 0): 0.0, (1, 0): 0.0},
            },
        },
        "TwoBody": {
            "Local": {
                "Parameter": "SlaterKanamori",
                "option": {
                    (0, (0,)): {"l": 0, "U": 2.0, "Up": 0.0},
                    (1, (0,)): {"l": 0, "U": 2.0, "Up": 0.0},
                },
            },
            "NonLocal": {
                ((0, 0), (1, 0)): {
                    0.20: [[0, 0, 0], [-1, 0, 0], [0, -1, 0]],
                },
            },
        },
    },
    "Control": {
        "Method": "gw",
        "Prefix": "my_calc",
        "NSCF": 20000,
        "Mix": 0.1,
        "T": 2000,
        "MatsubaraCutOff": 100,
        "ConstantW": 1.0,
    },
}

2. Run Simulation

Using the installed CLI command:

qassemble

You can also pass an explicit input file:

qassemble graphene_gw.in

Using Python module execution:

python -m QAssemble
python -m QAssemble graphene_gw.in

Directory Structure

QAssemble/
├── pyproject.toml              # Package configuration and dependencies
├── README.md
└── src/
    ├── QAssemble.py            # Legacy entry point (backward compatible)
    └── QAssemble/
        ├── __init__.py         # Package exports and version
        ├── __main__.py         # python -m QAssemble support
        ├── CLI.py              # CLI entry point (qassemble command)
        ├── Run.py              # Run class (input parsing and execution)
        ├── Crystal.py          # Lattice geometry, k-point grids, index mappings
        ├── CorrelationFunction.py  # Top-level workflow coordinator (TB / HF / GW)
        ├── FLatStc.py          # Static fermionic lattice (Hamiltonian, HF self-energy)
        ├── FLatDyn.py          # Dynamic fermionic lattice (Green's functions via DLR)
        ├── FLocStc.py          # Static fermionic local site
        ├── FLocDyn.py          # Dynamic fermionic local site
        ├── FPathStc.py         # Static fermionic path
        ├── FPathDyn.py         # Dynamic fermionic path
        ├── BLatStc.py          # Static bosonic lattice (bare/screened Coulomb)
        ├── BLatDyn.py          # Dynamic bosonic lattice (polarization, screened W)
        ├── BLocStc.py          # Static bosonic local site
        ├── BLocDyn.py          # Dynamic bosonic local site
        ├── BPathStc.py         # Static bosonic path
        ├── BPathDyn.py         # Dynamic bosonic path
        ├── Projector.py        # Projection utilities
        └── utility/
            ├── DLR.py          # Discrete Lehmann Representation transforms
            ├── Dyson.py        # Dyson equation solver
            ├── Fourier.py      # Lattice Fourier transforms
            ├── Common.py       # Shared utilities
            ├── Bare.py         # Bare Green's functions
            └── Mixing.py       # Mixing parameter control

Module Naming Convention

Prefix	Meaning
F	Fermionic
B	Bosonic
Lat	Lattice
Loc	Local
Path	Path
Stc	Static
Dyn	Dynamic

Configuration Reference

Section	Key	Description
Control	Method	Calculation type: "tb", "hf", "gw"
Control	Mode	"FromScratch" or "Restart"
Control	Prefix	Output HDF5 filename prefix
Control	NSCF	Max SCF iterations
Control	Mix	Mixing parameter for self-consistency
Control	T	Temperature in Kelvin
Control	MatsubaraCutOff	Matsubara frequency cutoff
Control	ConstantW	Constant W parameter for GW
Crystal	RVec	3x3 lattice vectors
Crystal	Basis	Basis atom positions and orbital counts
Crystal	KGrid	k-point grid [Nx, Ny, Nz]
Crystal	NElec	Number of electrons per spin
Crystal	SOC	Enable spin-orbit coupling
Crystal	NSpin	Number of spin channels
Crystal	CorF	Coordinate type: "F" (fractional) or "C" (Cartesian)
Hamiltonian	Hopping	One-body hopping terms
Hamiltonian	Onsite	On-site energy terms
Hamiltonian	Parameter	Coulomb parameterization: "SlaterKanamori", "Slater", "Kanamori"