Multi-Reference Configuration Interaction Calculations with Large Active Spaces Based on Matrix Product States for Simulating X-ray Absorption Spectra.

X-ray absorption spectroscopy (XAS) is widely used as an element-specific probe of local electronic and geometric structures, yet its accurate simulation for correlated systems remains challenging because it requires the simultaneous treatment of large active spaces and dynamic electron correlation. In this work, we present a matrix-product-state-based multireference configuration interaction (MPS-MRCI) approach for XAS simulations in such systems. The method is assessed across closed-shell, strongly correlated, and open-shell systems. For pyrazine, systematic calculations reveal the influence of active space size and dynamic correlation on the C and N K-edge spectra. Applications to pentacene, ozone, and the allyl radical further evaluate the performance of MPS-MRCI for a large π-conjugated system, a strongly correlated biradicaloid, and an open-shell system, respectively. The calculated spectra generally reproduce the major experimental features, with natural transition orbital analysis providing insight into the corresponding core excitations through visualization of hole-particle pairs. This work establishes MPS-MRCI as a viable approach for core-level spectroscopy of correlated systems, enabling the treatment of large active spaces and the recovery of dynamic correlation.