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Analytic First-Order Derivatives of (X)MS, XDW, and RMS Variants of the CASPT2 and RASPT2 Methods.

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Accurately characterizing electronic state crossings is challenging. This study develops analytic derivatives for CASPT2 and RASPT2 variants, finding rotated multistate CASPT2 (RMS-CASPT2) offers the most reliable results for conical intersection calculations.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Complex electronic structures in state crossings pose challenges for accurate characterization.
  • Locating minimum energy conical intersections is crucial for understanding photochemical and photophysical processes.

Purpose of the Study:

  • To develop and apply analytic derivatives of complete active space second-order perturbation theory (CASPT2) and restricted active space (RASPT2) variants.
  • To assess the performance of different CASPT2 variants in locating minimum energy conical intersections.
  • To compare the results obtained from CASPT2 and reference self-consistent field (SCF) calculations.

Main Methods:

  • Development of analytic derivatives for three CASPT2 variants and one RASPT2 variant.
  • Application of these methods to locate minimum energy conical intersections.
  • Comparison of results from different CASPT2 variants and reference SCF calculations.

Main Results:

  • The three CASPT2 variants yielded qualitatively similar results for conical intersection calculations.
  • The rotated multistate CASPT2 (RMS-CASPT2) variant demonstrated the least sensitivity to the number of states included.
  • Significant differences in energetics and bond lengths were observed between CASPT2 and reference SCF calculations.

Conclusions:

  • Analytic derivatives of CASPT2 and RASPT2 are effective tools for studying conical intersections.
  • RMS-CASPT2 is a robust method for conical intersection calculations, showing stability with varying numbers of states.
  • The choice of electronic structure method (CASPT2 vs. SCF) significantly impacts the predicted energetics and geometries.