Our
reduced-cost scheme [J. Chem. Phys.2018,148, 094111] based on the frozen
virtual
natural orbital and natural auxiliary function approaches is extended
to core excitations. The efficiency of the approximation is presented
for the second-order algebraic-diagrammatic construction [ADC(2)]
method invoking the core–valence separation (CVS) and density
fitting approaches. The errors introduced by the present scheme are
comprehensively analyzed for more than 200 excitation energies and
80 oscillator strengths, including C, N, and O K-edge excitations,
as well as 1s→ π* and Rydberg transitions.
Our results show that significant savings can be gained in computational
requirements at the expense of a moderate error. That is, the mean
absolute error for the excitation energies, being lower than 0.20
eV, is an order of magnitude smaller than the intrinsic error of CVS-ADC(2),
while the mean relative error for the oscillator strengths is between
0.06 and 0.08, which is still acceptable. As significant differences
for different types of excitations cannot be observed, the robustness
of the approximation is also demonstrated. The improvements in the
computational requirements are measured for extended molecules. In
this case, an overall 7-fold speedup is obtained in the wall-clock
times, while dramatic reductions in the memory requirements are also
achieved. In addition, it is also proved that the new approach enables
us to perform CVS-ADC(2) calculations within reasonable runtime for
systems of 100 atoms using reliable basis sets.