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.
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| http://dx.doi.org/10.1021/acs.jctc.2c00368 | DOI Listing |
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