A comprehensive benchmark of the XMS-CASPT2 method for the photochemistry of a retinal chromophore model

Saumik Sen, Igor Schapiro*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


The performance of the extended multi-state (XMS)-complete active space second-order perturbation theory (CASPT2) method has been assessed for the benchmark of a truncated retinal model, the penta-2,4-dieniminium cation (PSB3). This benchmark presents a challenge for multireference electronic structure methods because the wave function character is changing considerably. The assessment comprises ground and excited state pathways of the isomerisation, including transition states and conical intersection (CI) points. It also includes circular paths centred around different CIs, and 2D potential energy scans located in the branching planes. In this work, we compare the performance of the previous formulations of CASPT2, the single-state and the multi-state, with the recently developed XMS-CASPT2. Besides, we have also tested two variants of internal contraction in XMS-CASPT2, namely, the single-state single reference (SS-SR) and multi-state multireference (MS-MR) schemes. In our study, we find that XMS-CASPT2 corrects the artefacts and discontinuities present in other CASPT2 variants. The investigation of a circular loop and 2D potential energy surfaces around the surface crossing point shows that XMS-CASPT2 exhibits a smooth topology at the CI with the correct degeneracy. It also agrees better with the reference method MRCISD+Q in regions of the potential energy surfaces further away from CIs. Another observation is the close agreement between the results from the SS-SR contraction scheme and the more demanding MS-MR scheme.

Original languageAmerican English
Pages (from-to)2571-2582
Number of pages12
JournalMolecular Physics
Issue number19-20
StatePublished - 18 Oct 2018

Bibliographical note

Funding Information:
I.S. gratefully acknowledges funding by European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Program (grant agreement No. 678169 ‘PhotoMutant’).

Publisher Copyright:
© 2018, © 2018 Informa UK Limited, trading as Taylor & Francis Group.


  • PSB3
  • branching plane
  • conical intersection
  • excited state
  • isomerisation
  • retinal


Dive into the research topics of 'A comprehensive benchmark of the XMS-CASPT2 method for the photochemistry of a retinal chromophore model'. Together they form a unique fingerprint.

Cite this