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Implement the Lanczos algorithm in q-space (Bloch basis) where momentum
conservation q1+q2=q_pert makes the 2-phonon sector block-diagonal,
reducing psi vector size and anharmonic computation cost by ~N_cell.
New files:
- Modules/QSpaceLanczos.py: Python class inheriting from DL.Lanczos with
Hermitian Lanczos iteration, Bloch-transformed ensemble, q-pair mapping,
and correct PG symmetry construction (Cartesian rotations from spglib
fractional coords, manual atom permutation with lattice reduction)
- Modules/tdscha_qspace.jl: Julia extension for q-space ensemble averaging
with separate D3/D4 functions and acoustic mode masking
- tests/test_qspace/test_qspace_lanczos.py: Test comparing q-space vs
real-space Green functions (match within 2.4e-7 relative error)
Key fixes during development:
- Convert spglib rotations from crystal to Cartesian coords via M@R@M^{-1}
- Replace CC.symmetries.GetIRT (gives wrong atom permutations for some
symmetries) with manual modular lattice reduction
- Include translation in Bloch phase: exp(-2πi q'·L) where L=Rτ+t-τ'
- Mask acoustic modes (ω<1e-6) in f_Y, f_psi, and chi factors
Also includes the translational projection optimization in DynamicalLanczos
(O(n^2) permutations instead of O(n^3) matmuls for gamma_only).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
New usage guide section covering the QSpaceLanczos workflow (perturbation setup, mode looping, when to prefer q-space over real-space) and a new API reference page with auto-generated docs via mkdocstrings. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Computes the anharmonic phonon spectral function at the Brillouin zone boundary (X point) for SnTe using the Q-space Lanczos algorithm with the pre-existing test ensemble. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This was referenced Mar 19, 2026
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Added a new TD-SCHA algorithm completely in q space.
This algorithm is N^2 faster than the original algorithm for the TDSCHA.
For now, we miss the Raman implementation and the 2Phonon implementation.
For the rest it seems to work correctly.