  We present an efficient implementation of multireference algebraic
diagrammatic construction theory (MR-ADC) for simulating core-ionized states
and X-ray photoelectron spectra (XPS). Taking advantage of spin adaptation,
automatic code generation, and density fitting, our implementation can perform
calculations for molecules with more than 1500 molecular orbitals,
incorporating static and dynamic correlation in the ground and excited
electronic states. We demonstrate the capabilities of MR-ADC methods by
simulating the XPS spectra of substituted ferrocene complexes and azobenzene
isomers. For the ground electronic states of these molecules, the XPS spectra
computed using the extended second-order MR-ADC method (MR-ADC(2)-X) are in a
very good agreement with available experimental results. We further show that
MR-ADC can be used as a tool for interpreting or predicting the results of
time-resolved XPS measurements by simulating the core ionization spectra of
azobenzene along its photoisomerization, including the XPS signatures of
excited states and the minimum energy conical intersection. This work is the
first in a series of publications reporting the efficient implementations of
MR-ADC methods.
