3.36 Interface to LibRPA

The file-based interface is enabled by the tag output librpa in control.in. It is used when FHI-aims prepares input files for a separate LibRPA driver calculation. This export mode requires one of the standard MBPT requests

• •

  total_energy_method rpa, or

• •

  qpe_calc gw_expt.

When this tag is present, FHI-aims writes the structure, Brillouin-zone sampling, Kohn-Sham eigenvalues and eigenvectors, LRI triple coefficients, Coulomb matrices, and, for GW calculations, the exchange-correlation matrix elements and dielectric-function data required by the LibRPA driver. The optional subtag version:<n> selects the dataset format. version:0 is the default and writes the legacy LibRPA file formats. version:1 writes the LibRPA reader-v1 formats for the LRI triple coefficients, SCF Kohn-Sham eigenvectors, and Coulomb matrices. In this mode FHI-aims writes binary v1 Cs_data* files, binary KS_eigenvector_<rank>.dat files, and one Coulomb file per q-point, coulomb_mat_<iq>.dat and, for GW, coulomb_cut_<iq>.dat. LibRPA will detect output formats to choose the correct readers detects v1 files from their header. version:-1, or the compatibility alias legacy_v030, selects the old v0.3.0 layout. The real-space LRI coefficients are folded to the Born-von Karman cell by default; the old fold_C subtag is accepted only as a deprecated compatibility alias.

For periodic GW calculations along a band path, also specify output band; FHI-aims then exports the band-path eigenvalues, eigenvectors, and exchange-correlation matrix elements read by the LibRPA g0w0_band and exx_band driver tasks. If include_spin_orbit is used, FHI-aims exports the perturbatively spin-orbit-coupled spinor wave functions; the corresponding LibRPA driver input must then set use_spinor_wfc = t.

If the structure, basis, and k-point sampling are unchanged and only the exchange-correlation functional is modified, the subtag ks_only can be added to output librpa. This is a convenience alias for noC and noV: the real-space LRI triple coefficients and Coulomb matrices are suppressed, while the Kohn-Sham-dependent files, the dielectric function, and any requested band-path data are exported as usual.

For periodic GW calculations with KS_method serial, the subtags mommat and mommat:ks write momentum matrices in the Kohn–Sham basis, while mommat:nao writes the corresponding full Bloch-NAO matrices. The selectors are independent and may be specified together. Both representations contain matrix elements of the gradient operator,

without the factor $-i$ needed to form the physical momentum operator. They are related by $D_{\mathrm{KS},s}^{\alpha }=C_s^{†}{D}_{\mathrm{NAO}}^{\alpha }{C}_s$. With spin–orbit coupling, the NAO matrix is spin-independent and represents $D_{\mathrm{NAO}}^{\alpha }\otimes I_2$.

For each k-point, the KS selector writes mommat_ks_kpt_######.dat. Its unformatted stream starts with the six default-integer values i_k_point, n_state_min, n_state_max, ld, n_spin, and 3, followed by the complex*16 packed lower triangle for every spin and Cartesian direction. The NAO selector writes mommat_nao_kpt_######.dat. Its header contains the four default-integer values i_k_point, n_basis, n_basis*n_basis, and 3, followed by the full complex*16 $D_x$, $D_y$, and $D_z$ matrices in Fortran column-major order.

For example, both representations can be requested with

output librpa mommat:ks mommat:nao

LibRPA can also be called directly through its API from within FHI-aims. In this mode, FHI-aims passes the required data to LibRPA in memory and retrieves the computed quantities within the same run. No intermediate LibRPA dataset or librpa.in file is needed.

The API backend currently covers direct RPA correlation energies and periodic $G_0{W}_0$ calculations on the regular k-grid and, when output band is requested, on band-path k-points. For GW band output, FHI-aims writes GW_band1xxx.out files using the same band-path convention as the native periodic GW workflow. If output gw_nscf_exx_only_band is also set, the non-self-consistent EXX-only band correction is written to Non-SCF_EXX_only_band1xxx.out.

LibRPA band structures are obtained by interpolation from regular-k-grid quantities to the requested band path. Increasing the number of k-points along the band path, or adding more path segments, therefore has only a small additional cost compared with the main regular-k-grid G0W0 calculation. Current LibRPA band interpolation uses Wigner-Seitz Born-von Karman remapping by default.