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Computation of conical intersections by using perturbation techniques.

Luis Serrano-Andrés1, Manuela Merchán, Roland Lindh

  • 1Instituto de Ciencia Molecular, Universitat de València, Dr. Moliner 50, Burjassot, ES-46100 Valencia, Spain. Luis.Serrano@uv.es

The Journal of Chemical Physics
|April 20, 2005
PubMed
Summary
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This study explores multiconfigurational second-order perturbation theory (CASPT2) for finding electronic state crossings in molecules. It highlights challenges and validation methods for CASPT2 and multistate CASPT2 (MS-CASPT2) in complex systems.

Area of Science:

  • Quantum Chemistry
  • Theoretical Chemistry
  • Computational Chemistry

Background:

  • Electronic state crossings are crucial for understanding chemical reactions and photophysics.
  • Accurate theoretical methods are needed to locate these conical intersections and seams.
  • Multiconfigurational second-order perturbation theory (CASPT2) offers a balance of accuracy and efficiency.

Purpose of the Study:

  • To investigate the utility of CASPT2 and multistate CASPT2 (MS-CASPT2) for locating minima on seams of electronic state crossings.
  • To assess the performance and identify challenges associated with these methods, particularly the nonorthogonality issue in single-state CASPT2.
  • To compare CASPT2 results with established methods like complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI).

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Main Methods:

  • Application of single-state CASPT2 and MS-CASPT2 to identify minima on seams of crossing potential energy surfaces.
  • Analysis of the nonorthogonality problem in single-state CASPT2 solutions.
  • Comparative calculations with CASSCF and MRCI methods.
  • Illustrative calculations on molecular systems including LiF, formaldehyde, ethene dimer, and penta-2,4-dieniminium cation.

Main Results:

  • CASPT2 and MS-CASPT2 successfully locate minima on seams of electronic state crossings in the studied systems.
  • The nonorthogonality of single-state CASPT2 solutions presents a challenge that requires careful consideration.
  • Comparison with CASSCF and MRCI validates the general accuracy of the CASPT2 approach for these problems.
  • The application of MS-CASPT2 is shown to be sensitive to the quality of the reference wave function and active space size.

Conclusions:

  • CASPT2 is a viable method for exploring electronic state crossings, with practical validation procedures developed for polyatomic systems.
  • MS-CASPT2 is a powerful tool but demands rigorous analysis of result stability concerning the reference wave function and active space.
  • Further development and careful application are needed to fully leverage MS-CASPT2 for complex molecular systems.