Abstract
We present a theoretical model to compute the accurate photoionization dynamical parameters (cross-sections, asymmetry parameters and orbital, or cross-section, ratios) from Dyson orbitals obtained with the multi-state complete active space perturbation theory to the second order (MS-CASPT2) method. Our new implementation of Dyson orbitals in OpenMolcas takes advantage of the full Abelian symmetry point group and has the corrected normalization. The Dyson orbitals are coupled to an accurate description of the electronic continuum obtained with a multicentric B-spline basis at the DFT and TD-DFT levels. Two prototype diatomic molecules, i.e., CS and SiS, have been chosen due to their smallness, which hides important correlation effects. These effects manifest themselves in the appearance of well-characterized isolated satellite bands in the middle of the valence region. The rich satellite structures make CS and SiS the perfect candidates for a computational study based on our highly accurate MS-CASPT2/B-spline TD-DFT protocol.
Highlights
The electronic structure problem is at the core of quantum chemistry, and the heart of most advanced simulation tools
Only the ionization energies (IEs) were corrected with the state-specific NEVPT2 method, the Dyson orbitals—obtained at the CASSCF level—were missing important correlation description on the bound states, which can affect to some extent the photoionization properties
Both IEs and Dyson amplitudes are corrected with the MS-CASPT2 method and the continuum is further extended to TD-density functional theory (DFT), providing a better description of correlation in the photoionization dynamical parameters
Summary
The electronic structure problem is at the core of quantum chemistry, and the heart of most advanced simulation tools. Impressive advances have been brought over the years so that practical “black-box” tools are currently available to the general chemist. Despite the satisfactory performance of standard approaches, especially those based on density functional theory (DFT), in many applications the most accurate methods are based on correlated ab initio formalisms, the only ones that can address complex multiconfigurational situations. Notwithstanding the great advances in the treatment of the correlation problem, it remains the stumbling block of ab initio approaches and continues to be intensely researched. Like current coupled-cluster (CC) [4], are best at the dynamical problem, while multiconfigurational approaches [3] treat the quasidegeneracies well, but are less effective in describing the dynamical contribution. Even the adequate definition of the quasidegenerate space is often not trivial, and it requires an understanding of the basic features of the problem considered
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