Learning the self-energy through equivariant neural networks
The electronic self-energy determines the single-particle Green's function and, through it, quasiparticle energies and spectral functions, but computing it from first principles is expensive and it is a difficult machine-learning target: complex-valued, matrix-valued in a local-orbital basis, and dependent on a continuous frequency. We predict the dynamical correlation part of the imaginary-frequency G_0W_0 self-energy matrix of molecules from atomic species and coordinates with an E(3)-equivariant message-passing network, the rotational covariance of the orbital-space matrix being imposed by the architecture itself and the static exchange part taken from the density-functional reference. Frequency enters either as an explicit network input or through frequency-specific output heads, and the predicted matrices are continued to the real axis in a representation in which causality and a non-negative spectral density hold by construction. The frequency-conditioned model reproduces matrix elements to below 1 meV on the imaginary axis for the systems studied here, and that accuracy survives the continuation: for water clusters, including a tetramer absent from the training set, and for chemically diverse molecules from QM9, the reconstructed spectral functions follow the G_0W_0 reference peak by peak and the frontier quasiparticle levels are recovered to a few tens of meV. Learning the self-energy on the imaginary axis is therefore a practical route to real-frequency spectra: once the model is trained, and given the density-functional reference it is built on, a new geometry costs no explicit G_0W_0 calculation.