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Marta Gałyńska1, Katharina Boguslawski1

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Accurate ionization potentials (IPs) require accounting for dynamical correlation. This study shows that triple excitations are crucial for chemical accuracy in IP calculations, regardless of molecular orbital basis choice.

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Area of Science:

  • Quantum chemistry
  • Computational chemistry
  • Chemical physics

Background:

  • Ionization potentials (IPs) are key chemical reactivity indicators and vital for technological applications.
  • Seniority-zero coupled-cluster (CC) methods lack dynamical correlation, leading to significant errors in IP predictions (approx. 1.5 eV).

Purpose of the Study:

  • To investigate the impact of dynamical correlation and molecular orbital (MO) basis sets on IP calculations.
  • To evaluate pCCD-based methods, IP-EOM-CCD, and IP-EOM-CCSD against high-level CC methods and experimental data for 230 ionized states in 70 molecules.

Main Methods:

  • Equation-of-motion coupled-cluster (IP-EOM-CC) methods, including pCCD, IP-EOM-CCD, and IP-EOM-CCSD.
  • Comparison with CCSD(T) or CCSDT reference data and experimental IPs.
  • Analysis of canonical vs. localized MOs and the role of dynamical correlation.

Main Results:

  • All investigated frozen-pair CC methods showed similar performance, with errors within chemical accuracy.
  • The choice of MO basis (canonical vs. natural pCCD-optimized) had a marginal effect on IPs when dynamical correlation was included.
  • Dynamical correlation significantly improves IP accuracy compared to seniority-zero pCCD.

Conclusions:

  • Triple excitations are essential for achieving chemical accuracy in IPs using pCCD-based methods.
  • Accounting for dynamical correlation is more critical than MO basis choice for accurate IP calculations.