An Improved Virtual Orbital Driven Similarity Renormalization Group Approach for Core-Ionized and Core-Excited States.

This work combines the multireference driven similarity renormalization group (DSRG) with a reference state obtained using improved virtual orbitals (IVOs) and generalized active space configuration interaction (GASCI) to model core-ionized and core-excited states without costly orbital optimizations. We test the accuracy of the resulting IVO-GASCI-DSRG method combined with three truncation levels across four data sets of molecules containing first-row elements (small molecules, potential energy surfaces, small-to-medium molecules, and X-ray absorption spectra). It is found that the IVO-GASCI-DSRG approach with an active space consisting of three GAS spaces and third-order perturbative corrections (IVO-GASCI[3]-DSRG-MRPT3) strikes the best balance between cost and accuracy. This method exhibits good agreement with the most accurate DSRG truncation scheme based on self-consistent orbitals on small-molecule benchmarks, and it is capable of accurately predicting the potential energy surfaces of core-excited and core-ionized states of CO, N2, and HF. To demonstrate the applicability of this method to medium-sized molecules, we simulate the X-ray absorption spectra of thymine and adenine using IVO-GASCI-DSRG-MRPT3, successfully reproducing key experimental spectral features.