Among the many options of multireference electron correlation methods, approaches based on second-order perturbation theory (PT2) with a multiconfigurational reference function offer an appealing compromise between accuracy and computational complexity. Their popularity is reflected by the large number of available flavors, which typically differ in the partitioning of the Hamiltonian, the many-electron basis used to express the first-order wave function or the conditions to obtain the expansion coefficients. One of the most known and used of these multireference perturbation theory (MRPT) approaches is complete active space PT2 (CASPT2), whose development has seen some important activity in recent times, with a new modified zeroth-order Hamiltonian (CASPT2-K), new quasi-degenerate variants (XDW-CASPT2 and RMS-CASPT2), reduced scaling implementations, and analytic nuclear energy gradients and derivative couplings. Despite its popularity and general applicability, CASPT2 suffers from an issue common to other MRPT-based approaches, the intruder state problem (ISP). While this generally does not appear when modeling the ground state of small organic compounds, it is much more common in transition metal complexes and excited states applications. Two techniques have been introduced during the years to avoid this issue in CASPT2, namely the real and imaginary level shifts. In both cases, the idea is to add a uniform shift to the resolvent operator, avoiding the singularity that causes the ISP. As a consequence, these techniques introduce a dependence of the results on a user-defined parameter, which should ideally be minimal, especially in absence of intruder states. In other words, the results for well-behaved cases should be as insensitive as possible to the value of the input parameter. This is because it is common that, for a molecular system, only a subset of electronic states or conformations is affected by the ISP, but all of them are treated with the same intruder-state-removal technique. In principle, the results of well-behaved cases should not change as a function of the parameter, as they did not require any intervention in the first place. This is for instance typical in the calculation of vertical transition energies, where only one or a few states of the excitation manifold is plagued by ISPs, albeit imposing the use of the shift for all states. Furthermore, considering the current wide availability of analytic nuclear energy gradients for CASPT2, exploration of the potential energy surfaces, especially in the excited states, has become more common, requiring a robust approach that effectively avoids ISPs, and that at the same time does not affect the qualitative description in regions of the PES where these are not present. In this context, the real level shift is not an ideal solution, reason for which the imaginary level shift had been developed. This second option is much better, because it selectively corrects large amplitudes. Nevertheless, the results obtained remain susceptible to the choice of input parameter. In an attempt to find a new intruder-state-removal technique that is more insensitive to this, but equally effective in removing the singularities in the first-order wave function, we propose to use σ p regularization in CASPT2. This approach is a simple and effective way to remove the ISP, and that from a theoretical perspective appears to have some advantages over the real and imaginary level shift techniques. In this contribution we introduce the σ p -CASPT2 method and critically analyze it in comparison to the established alternatives.