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

Abstract

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.

Ultrafast and radiationless electronic excited state decay of uracil and thymine cations: computing the effects of dynamic electron correlation

“…XMS-CASPT2 is shown to be blue-shifted with respect to MS and single-state formulations, a feature previously reported for the singlet manifold. 55,75 In contrast, CASSCF places the first three vertical and first adiabatic ionisation potentials at 8.58, 9.36, 9.47 and 8.18 eV, respectively (Table 1), which deviate from the experimental evidence by ∼1 eV. Nevertheless, CASSCF still provides acceptable energies when considering solely the cationic manifold and the differences among the diverse cationic states.…”

“…crossings between ionic and covalent states, as reported by Serrano-Andrés et al, 53 and this is unfeasible for optimisations; and iii) XMS-CASPT2 has been recently shown to provide smoother potential energy surface profiles around conical intersections due to treating crossings between ionic and covalent states more accurately than SS-and MS-CASPT2 formulations. 54,55 Additional computations averaging over the lowest-lying 30 electronic doublet states were carried out on top of the different minima to evaluate the excited state absorption signals. 56,57 Our simulations of these spectral signals assume that excited state absorptions of the individual 2 n + O and 2 π + states are dominated by the electronic structure at their corresponding minima, 58,59 thus neglecting the time-evolution of the system and its lineshape as their simulation requires more sophisticated approaches that are out of the scope of the present study.…”

Computing the Ultrafast and Radiationless Electronic Excited State Decay of Cytosine and 5‐methyl‐cytosine Cations: Uncovering the Role of Dynamic Electron Correlation

“…For presenting and discussing the energies at the FC region, we have chosen to average over the different CASPT2 formulations as this allows us to show the mean value as well as the standard deviation expected by modifying the zeroth‐order Hamiltonian. However, for geometries and energies away from the FC region we have only reported XMS‐CASPT2 estimates as this has been shown to provide a better balance in the simultaneous description of covalent and ionic excited states and therefore gives us more reliable geometries, particularly for crossing regions . It is worth noting that the IPEA shift described above was calibrated for the single‐state formulation of CASPT2, 0.25 a.u.…”

Four resonance structures elucidate double-bond isomerisation of a biological chromophore

“…The b-CoIn branching plane of pCTM À is similar to that previously reported for the double-bond isomerisation of the retinal protonated Schiff base (RPSB) chromophore and analogues in rhodopsins. 60,61 Fig . 6 Characterisation of the b-twisted CoIn: (a) the branching plane vectors x 1 and x 2 for model 3 (see also Fig.…”