Recently, it was shown that the calculation of quasiparticle energies using the G0W0 approximation can be performed without computing explicitly any virtual electronic states, by expanding the Green function and screened Coulomb interaction in terms of the eigenstates of the static dielectric matrix. Avoiding the evaluation of virtual electronic states leads to improved efficiency and ease of convergence of G0W0 calculations. Here, we propose a further improvement of the efficiency of these calculations, based on an approximation of density-density response functions of molecules and solids. The approximation relies on the calculation of a subset of eigenvectors of the dielectric matrix using the kinetic operator instead of the full Hamiltonian, and it does not lead to any substantial loss of accuracy for the quasiparticle energies. The computational savings introduced by this approximation depend on the system, and they become more substantial as the number of electrons increases.
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Here, we did not attempt to solve the problem on how to accurately compute resonant molecular energy levels: our goal is to compare results obtained with solutions of the Sternheimer equation using the full Hamiltonian [Eq. (1)] and approximate solutions using only the kinetic operator [Eq. (4)]. As long as the results obtained with the two procedures agree, we consider the results obtained from Eq. (4) as accurate.