The paper is organized as follows. In the next section we present the theoretical foundations of the ISP and the various intruder-state-removal techniques. We then conduct a series of tests to evaluate the effectiveness of σ p -CASPT2 in removing the ISPs and study the sensitivity of this new approach with respect to the input parameter. In the last section we summarize our findings and briefly discuss the remaining open issues in this context and possible future directions to solve them.