Abstract
Herewith, we propose two new exponents for the recently introduced XDW-CASPT2 method [S. Battaglia and R. Lindh, J. Chem. Theory Comput. 16, 1555-1567 (2020)], which fix one of the largest issues hindering this approach. By using the first-order effective Hamiltonian coupling elements, the weighting scheme implicitly takes into account the symmetry of the states, thereby averaging Fock operators only if the zeroth-order wave functions interact with each other. The use of Hamiltonian couplings also provides a physically sounder approach to quantitate the relative weights; however, it introduces new difficulties when these rapidly die off to zero. The improved XDW-CASPT2 method is critically tested on several systems of photochemical relevance, and it is shown that it succeeds in its original intent of maintaining MS-CASPT2 accuracy for the evaluation of transition energies and at the same time providing smooth potential energy surfaces around near-degenerate points akin to XMS-CASPT2.
Highlights
The development of multireference approaches is an active field of research, in particular, in the context of excited state chemistry.1 Thanks to ever increasing computational resources and method developments, it is possible to routinely investigate photochemical processes of general interest, with applications tackling important technological challenges such as devising efficient photovoltaic materials, improving photodynamic therapies, and developing bioimaging probes.2 Among the many possible options, multireference perturbation theory (MRPT) is a widely used approach that provides an appealing accuracy–computational cost trade-off
The novelty of XDWCASPT2 relies on the definition of the state-specific Fock operators, whose construction is achieved through (1) a new set of multireference wave functions within the model space and (2) a dynamical weighting scheme that changes as the zeroth-order wave functions change with, for example, the molecular geometry
We test the new exponents on several exemplary systems that cover important use cases in photochemistry: vertical transition energies, symmetry-breaking potential energy surfaces, exactly degenerate states, and conical intersections
Summary
The development of multireference approaches is an active field of research, in particular, in the context of excited state chemistry. Thanks to ever increasing computational resources and method developments, it is possible to routinely investigate photochemical processes of general interest, with applications tackling important technological challenges such as devising efficient photovoltaic materials, improving photodynamic therapies, and developing bioimaging probes. Among the many possible options, multireference perturbation theory (MRPT) is a widely used approach that provides an appealing accuracy–computational cost trade-off. The recently introduced extended dynamically weighted complete active space second-order perturbation theory (XDWCASPT2) method is a new variant of the well-known CASPT2 approach, which essentially interpolates between multistate CASPT2 (MS-CASPT2) and extended MS-CASPT2 (XMSCASPT2), retaining crucial features from both: an accurate zeroth-order description of individual states and smooth potential energy surfaces (PESs) in regions of electronic near-degeneracy.
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