Release Notes for TDSCHA v1.6.1

Release Date: March 25, 2026

This release introduces major performance optimizations and new computational capabilities, most notably the q-space implementations that dramatically accelerate calculations for periodic systems.

Major New Features

1. QSpaceLanczos: Q-Space Spectral Calculations

A new module implementing the Lanczos algorithm in the Bloch (q-space) basis, exploiting momentum conservation for massive performance gains.

Key Features:

Momentum Conservation: The 2-phonon sector is block-diagonal with pairs satisfying q₁ + q₂ = q_pert, reducing the ψ-vector size by ~N_cell compared to real-space
Hermitian Lanczos: Uses complex arithmetic with Hermitian inner products for proper convergence
Julia Backend: High-performance implementation via Modules/tdscha_qspace.jl
Bloch-Transformed Ensemble: Direct q-space ensemble averaging without real-space supercell overhead

Performance Impact:

Memory reduction: O(N_modes²) → O(n_bands²) for the 2-phonon sector
Speedup scales as ~N_cell for spectral calculations
Essential for large supercells where real-space Lanczos becomes prohibitive

Usage:

import tdscha.QSpaceLanczos as QL

# Initialize q-space Lanczos solver
qlanc = QL.QSpaceLanczos(ensemble)
qlanc.init(use_symmetries=True)

# Compute spectral function at specific q-point
qlanc.prepare_q(0)  # Gamma point
qlanc.run_FT(n_steps=1000)
green = qlanc.get_green_function_continued_fraction(energies)

Files Added:

Modules/QSpaceLanczos.py (1,232 lines)
Modules/tdscha_qspace.jl (529 lines)
tests/test_qspace/test_qspace_lanczos.py (273 lines)

2. QSpaceHessian: Fast Free Energy Hessian

The fastest and most memory-efficient method in the SSCHA ecosystem for computing the full anharmonic free energy Hessian, including fourth-order contributions.

Key Features:

Block-Diagonal Structure: Translational symmetry makes the Liouvillian block-diagonal in q-space
Iterative Solver: GMRES with harmonic preconditioner; BiCGSTAB fallback for difficult cases
Symmetry Exploitation: Point-group symmetries reduce computation to the irreducible q-set
Hermiticity Enforcement: Explicit symmetrization ensures real eigenvalues
Anharmonic Control: ignore_v3 and ignore_v4 flags to exclude cubic/quartic terms

Performance Impact:

Metric	Direct (`get_free_energy_hessian`)	QSpaceHessian
Memory	O(N_modes⁴)	O(n_bands⁴)
Time	O(N_modes⁶)	O(n_bands³ × N_q,irr)
Speedup (4×4×4 cell)	1×	~65,000×

For a 4×4×4 supercell (N_cell = 64, N_q,irr ≈ 4), calculations that would require terabytes of RAM are now feasible on a laptop.

Usage:

import tdscha.QSpaceHessian as QH

# Compute full Hessian at all q-points
hess = QH.QSpaceHessian(ensemble)
hess.init()
hessian_dyn = hess.compute_full_hessian(tol=1e-6, max_iters=500)

# Or compute at single q-point
H_gamma = hess.compute_hessian_at_q(iq=0)

Advanced Features:

Dense Fallback: For problematic q-points near phase transitions (dense_fallback=True)
Acoustic Mode Handling: Proper null-space projection for non-self-conjugate q-points
Mode Degeneracy Exploitation: Schur's lemma reduces GMRES solves for degenerate modes

Files Added:

Modules/QSpaceHessian.py (701 lines)
tests/test_qspace/test_qspace_hessian.py (170 lines)
docs/qspace-hessian.md (comprehensive documentation)

3. Gamma-Only Optimization: 8× Speedup for Γ-Point Calculations

Major performance enhancement for Γ-point (q = 0) spectral and response calculations.

Optimizations:

Reduced Python-Julia Bridge Overhead: Minimized data marshalling for gamma-only mode
O(n²) Permutations: Replaced O(n³) matrix multiplications with direct index permutations in apply_L_translations
Symmetry-Aware: Exploits point-group symmetries for additional speedup

Impact:

8× speedup for gamma-point IR/Raman response calculations
Particularly effective for large supercells where the Python-Julia boundary crossing was a bottleneck
Maintains full accuracy (verified against non-optimized implementation)

Usage:
The optimization is automatic when:

The perturbation is at the Γ-point (q_pert = 0)
use_symmetries=True is set
The Julia backend is active (MODE_FAST_JULIA)

Files Modified:

Modules/DynamicalLanczos.py (gamma-only detection and optimization)
Modules/tdscha_core.jl (translational projection optimization)

Additional Improvements

Documentation Overhaul

New Documentation Site: Full mkdocs-based documentation at sscha.eu/tdscha
API Reference: Auto-generated from docstrings
Theory Guide: Mathematical background on q-space methods
Examples: SnTe spectral function at X point, IR response calculations

Command-Line Interface

New executable scripts for common workflows:

tdscha-convergence-analysis — Analyze Lanczos convergence
tdscha-plot-data — Plot spectral functions
tdscha-output2abc — Convert output formats
tdscha-hessian-convergence — Monitor Hessian convergence

Testing & Validation

Q-Space Correctness Tests: Verify q-space vs. real-space Green functions match within 2.4×10⁻⁷ relative error
Wigner Representation Tests: Compare Wigner vs. non-Wigner spectral functions
MPI Parallel Tests: CI-validated MPI correctness
IR Perturbation Tests: Modulus consistency between real-space and q-space Lanczos

Bug Fixes

Q-Point Mapping: Fixed errors for q-points where -q does not map to itself
Symmetry Construction: Corrected point-group symmetry handling in q-space (Cartesian rotations, atom permutations)
Acoustic Modes: Proper masking of acoustic modes (ω < 10⁻⁶) in ensemble averages
Version Synchronization: Aligned version numbers across pyproject.toml, meson.build, and git tags

Breaking Changes

None. This release is fully backward-compatible with v1.5.

Migration Guide

For Existing Users

No code changes required. Existing scripts will continue to work unchanged.

To benefit from new features:

For faster Hessian calculations: Replace ensemble.get_free_energy_hessian() with QSpaceHessian
For large-supercell spectra: Use QSpaceLanczos instead of Lanczos when memory is constrained
For Γ-point response: Ensure use_symmetries=True to activate 8× speedup automatically

Dependencies

No new required dependencies. Optional dependencies:

spglib — For symmetry exploitation (recommended)
julia — Required for q-space modules and gamma-only optimization
mpi4py — For MPI parallelization

Contributors

Lorenzo Monacelli (main development)
Claude Opus 4.6 (q-space implementation assistance)

References

R. Bianco, I. Errea, L. Paulatto, M. Calandra, and F. Mauri, Phys. Rev. B 96, 014111 (2017) — Free energy Hessian theory
L. Monacelli, Phys. Rev. B 112, 014109 (2025) — Static susceptibility formulation
L. Monacelli and F. Mauri, 2021 — Q-space Liouvillian block-diagonality

Full Changelog

See the git log for all 43 commits since v1.5.

Key Commits:

47ab893c — Gamma-only implementation with tests
31263868 — 8× speedup via reduced Python-Julia overhead
1a65d466 — QSpaceLanczos module with Bloch momentum conservation
b180a88f — QSpaceHessian for free energy Hessian in q-space
7cd005ef — Mode degeneracy exploitation (Schur's lemma)
a7ddc302 — Hermitian symmetry exploitation in q-pair blocks
ed693e2c — Raman perturbation methods for QSpaceLanczos

#Update TDSCHA to version 1.4

This new version include some bugfixes, especially in the StaticHessian calculation which has was broken in the previous release.
This version allows a seamless integration with julia, no more need to call python-jl, and automatic installation of julia libraries without the user having to care about anything.
Also, the fast julia mode is the default one, if available and not otherwise specified by the user.

We also changed the versioning, jumping from 1.0 to 1.4, to respect the same minor versioning of cellconstructor and python-sscha.

What's Changed

Lanc efficient symmetries by @mesonepigreco in #1
Julia exec by @mesonepigreco in #3

Full Changelog: https://github.com/SSCHAcode/tdscha/commits/v1.4

Releases: SSCHAcode/tdscha

1.6.1

Release Notes for TDSCHA v1.6.1

Major New Features

1. QSpaceLanczos: Q-Space Spectral Calculations

2. QSpaceHessian: Fast Free Energy Hessian

3. Gamma-Only Optimization: 8× Speedup for Γ-Point Calculations

Additional Improvements

Documentation Overhaul

Command-Line Interface

Testing & Validation

Bug Fixes

Breaking Changes

Migration Guide

For Existing Users

Dependencies

Contributors

References

Full Changelog

Uh oh!

1.5

Release Notes – Version 1.5

Highlights

New Features

What's Changed

Contributors

Uh oh!

Version 1.4

What's Changed

Contributors

Uh oh!