In this section we present a series of calculations to evaluate the effectiveness of σ p regularization in solving the intruder state problem in CASPT2, and compare it to the level shift techniques. We have implemented the σ 1 and σ 2 regularizers in a development branch of OpenMolcas and note that they work with any quasi-degenerate variant of CASPT2 currently available in the package. In the following, we shall refer to CASPT2 with σ p regularization simply as σ p -CASPT2.