Green's functions and response¶

For a given orbital reference, QuAcK can compute one-particle poles with Green's-function methods and two-particle excitation energies with response methods. See method availability before choosing a reference or a self-consistency scheme.

Family	Approximation or observable	Settings to record
GF2 and GF3	One-particle self-energy through second or third order, with additional variants in the drivers	Reference, self-consistency, regularization, linearization, and root-selection procedure
GW	Self-energy built from a Green's function and screened interaction	Reference, TDA for screening, regularization, and quasiparticle solver settings
T-matrix	Repeated particle–particle or electron–hole scattering	Channel, self-consistency, and screening or pair-space options
IP/EA-ADC and vertex corrections	One-particle poles with ADC or diagrammatic self-energies	Dyson or alternate formulation, diagonal approximation, and static self-energy options
phRPA and ppRPA	Neutral excitations and pair addition/removal poles, respectively	Channel, spin sector, TDA, and active orbital space
BSE	Neutral or charged two-particle response using a parent calculation	Parent self-energy, channel, static or dynamical kernel, and solver
Parquet	Coupled two-particle scattering channels and a one-particle self-energy	One- and two-body iteration counts, DIIS, channel TDA switches, and convergence

GF2 and GF3¶

GF2 uses a second-order self-energy. The G0F2, evGF2, qsGF2, and scGF2 flags select different levels of self-consistency. The third-order routes are G0F3, evGF3, and psdG0F3. See the catalogue for driver coverage.

Single-shot and self-consistent GW¶

Input flag	Scheme	Drivers
G0W0	Single-shot GW from the input orbitals and energies	R, U, G
evGW	Eigenvalue self-consistency with fixed orbitals	R, U, G
qsGW	Quasiparticle self-consistency with an updated effective Hamiltonian and orbitals	R, U, G
scGW	Fully self-consistent GW on the imaginary axis	R and a dedicated Bogoliubov branch

T-matrix¶

The particle–particle and electron–hole T-matrix families sum repeated scattering in their respective channels. The catalogue lists the single-shot, eigenvalue and quasiparticle self-consistent paths, and the upfolded particle–particle path.

ADC and vertex corrections¶

QuAcK includes IP/EA-ADC(2), IP/EA-ADC(3), SOSEX, 2SOSEX, and G3W2 variants. The catalogue lists the ADC representations and identifies generalized routes limited to the diagonal Dyson implementation.

Parquet¶

evParquet and qsParquet have restricted and generalized paths. GWParquet is a single-shot workflow for canonical RHF.

Experimental Faddeev¶

FTDA and FRPA are standalone experimental implementations for canonical RHF. The guide describes the pair kernels, couplings, and solver options.

Response options¶

The screening TDA switch TDA_W and the excitation-space TDA switch TDA control different problems. Likewise, phBSE and ppBSE request different channels. Read the corresponding parent-method routine before using a combination for production results. Record the active orbital space and any frozen-core or CVS settings together with the parent-method options.

Reading spectra¶

The quasiparticle equation can admit several roots. Save the pole energy and weight together, especially for inner-valence ionization. For an upfolded Hermitian problem, the weight comes from the squared norm in the one-particle block. A Davidson calculation targeting selected roots gives a subset of that spectrum.

BSE availability¶

These entries cover the standard real-reference drivers. They indicate active calls, with dynamical corrections listed separately where checked.

Parent	R	U	G
G0W0	ph and pp, both with optional dynamical correction	ph with optional dynamical correction	ph and pp. The ph dynamical call is inactive
evGW, qsGW	ph and pp, both with optional dynamical correction	ph with optional dynamical correction	ph. The ph dynamical call is inactive
G0F2	ph and pp	—	ph and pp
evGF2	ph and pp	—	ph
qsGF2	ph and pp	—	—
G0T0pp	ph and pp	—	ph and pp
evGTpp, qsGTpp	ph	—	—

The Fortran source index links to the parent and response routines. The table covers the listed families and standard references. CAP, complex references, and alternate workflows require their own checks.

The phBSE2 kernel switch is used inside the restricted GW particle–hole BSE routines. Set phBSE to enter that response path.