Regularized CASPT2: an Intruder-State-Free Approach

Stefano Battaglia; Lina Fransén; Ignacio Fdez Galván; Roland Lindh

doi:10.1021/acs.jctc.2c00368

Regularized CASPT2: an Intruder-State-Free Approach

J Chem Theory Comput. 2022 Aug 9;18(8):4814-4825. doi: 10.1021/acs.jctc.2c00368. Epub 2022 Jul 25.

Authors

Stefano Battaglia¹, Lina Fransén¹, Ignacio Fdez Galván¹, Roland Lindh¹

Affiliation

¹ Department of Chemistry─BMC, Uppsala University, P. O. Box 576, SE-75123Uppsala, Sweden.

Abstract

In this work we present a new approach to fix the intruder-state problem (ISP) in CASPT2 based on σ^p regularization. The resulting σ^p-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^p-CASPT2 in removing the ISP and the sensitivity of the approach with respect to the input parameter. We found that σ^p-CASPT2 compares favorably with respect to previous approaches and that different versions, σ¹-CASPT2 and σ²-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.