Incorporating the missing correlations in the DFT excitation spectra of atomic systems using an auxiliary boson system and diffusion Monte Carlo

We present a quantum Monte Carlo (QMC) framework for evaluating excitation spectra that replaces the traditional Jastrow factor with an auxiliary boson (AB) ground state. The AB corrected ground state is treated using diffusion Monte Carlo (DMC) and coupled to a fermionic component. This construction statistically incorporates many-body correlations that are typically approximated by empirical Jastrow factors, reducing errors associated with its basis truncation and mean-field limitations. Excitation energies are obtained by sampling fermion–boson coupling terms over the AB ground state, providing systematic corrections to approximate eigenstates derived from other ab-initio methods such as density functional theory (DFT). Building on prior model-system developments [1] and on the PYQMC code[2], we extend this approach to atomic-scale calculations and assess its ability to capture correlated excitations beyond DFT. Results for representative atomic and molecular systems will be presented, demonstrating how the AB-DMC formalism can improve the accuracy of excitation spectra from mean-field approaches.