X-ray photoelectron spectroscopy (XPS) has become a popular tool for the investigation of the chemical structure of molecules. Its ability to describe the local structure in a chemical system with atom-specific details has made it an important method for chemical investigations of molecules and surfaces . In recent times, there has been a remarkable increase in the utilization of X-ray spectroscopy. This is primarily attributed to the wider availability of high-energy X-ray light sources , making even table-top instruments readily accessible . Theoretical simulation can be a useful companion for the interpretation of experimentally measured XPS spectra. Among all the single-reference quantum chemical methods available for the simulation of experimental properties, the coupled cluster method has emerged as one of the most accurate and systematically improvable methods. The ionization potential of a molecule can be calculated using the coupled cluster method by two alternative approaches. One is the so-called ∆ based methods , where the total energy of the neutral and cation is calculated separately, and the IP value is calculated as their difference. On the other hand, one can directly calculate the ionization potential using the so-called equation of motion (EOM) approach. The Equation of motion coupled cluster (EOM-CC) approach gives an identical answer to the linear response coupled cluster method for excitation energies. Both approaches are similar to symmetryadapted cluster CI (SAC-CI) . The recent extension of core-valence separation techniques to EOM-CC has extended its applicability for core-level spectroscopy. However, the high computational cost inherent to the coupled cluster method restricts its applicability beyond small molecules. The EOMCC method is commonly used singles and doubles truncation of the cluster and the excitation operator (EOM-CCSD), which scales as iterative N 6 power of the basis set and has a similar storage requirement as that of the ground state coupled cluster method. The EOM part scales as N for the ionization problem and generally shows an error bar of 0.1 eV with respect to the theoretical best estimate . The situation is slightly different for coreionization energies as the ionization of core electron involves a high degree of orbital relaxation effect, which the linear excitation operator cannot properly include in the CCSD truncation . The IP-EOM-CCSD method shows an error bar in the range of 1.5-2 eV for core ionization energy with respect to theoretical best estimates . One needs to include higher-body excitation operators to get the desired accuracy in the simulated ionization potential. The full inclusion of three-body operators (EOM-CCSDT) will increase the scaling to N 8 , which will make the calculation infeasible beyond triatomic molecules in any reasonable basis set. Partial triples correction can be an economical compromise between cost and accuracy. Mathews and coworkers have shown that among the various triples correction schemes, the IP-EOM-CCSD* gives the best possible accuracy. However, even the computational cost of the IP-EOM-CCSD* method can be prohibitive for molecules beyond ten second-row elements unless one uses additional approximation or has access to a massively parallel code and supercomputing facility. The use of natural orbitals has become extremely popular in recent times to reduce the dimension of the virtual space in coupled cluster calculations . Mester and Kallay have reported a lower scaling ADC(2) method core excitation energy based on CIS(D) natural orbitals . Krylov and co-workers have used frozen natural orbital to calculate valence ionization energy using the IP-EOM-CCSD method. Koch and co-workers have reported a natural transition orbital(NTO) based multi-level coupled cluster method for core-excitation energy. Dutta and co-workers have reported a DLPNO framework for the IP-EOM-CCSD for valence and core-ionization energy. However, it has been observed that the treatment of core-ionization energy requires tighter thresholds than the corresponding valence ionization. In all the previous reports of the natural orbital-based IP-EOM-CCSD method, the ground state MP2 natural orbitals are used. Our recent study found that utilizing state-specific natural orbitals can offer a more balanced trade-off between computational efficiency and accuracy when predicting valence ionization energy within the framework of the Algebraic diagrammatic construction method. In this work, we extended the use of state-specific natural orbitals to the core-ionization problem.