Regularized CASPT2: an Intruder-State-Free Approach.

In this work we present a new approach to fix the intruder-state problem (ISP) in CASPT2 based on σ regularization. The resulting σ-CASPT2 method is compared to previous techniques, namely, the real and imaginary level shifts, on a theoretical basis and by performing a series of systematic calculations. The analysis is focused on two aspects, the effectiveness of σ-CASPT2 in removing the ISP and the sensitivity of the approach with respect to the input parameter. We found that σ-CASPT2 compares favorably with respect to previous approaches and that different versions, σ-CASPT2 and σ-CASPT2, have different potential application domains. This analysis also reveals the unsuitability of the real level shift technique as a general way to avoid the intruder-state problem.