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Abstract <p>The paper presents an implementation of the spin-unrestricted complete active space self-consistent field (UCASSCF) method. The key feature of this modification is the independent description of molecular orbitals for different spin projections: separate sets of spatial orbitals are used for α and β electrons, which provides a more flexible description of the multielectron wave function when taking into account the effects of spin polarization. Particular attention is paid to the analysis of the calculated expectation values of the spin-squared operator and their deviations from the exact values for a given multiplet state. The method was tested by calculating the potential energy curve of the ground state of the CN radical. It is shown that, in contrast to the UHF and UCASCI methods, spin contamination of the UCASSCF wave function is negligible in the region of the equilibrium geometry configuration of the radical. In this case, the total energy turns out to be systematically lower than that of the restricted version and the energy contribution from spin polarization is 0.3 eV. The development of unrestricted multiconfiguration quantum chemistry methods is essential for modeling the electronic structure of systems with significant spin polarization, including atoms and molecules in strong magnetic fields.</p>

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An Unrestricted Complete Active Space Self-consistent Field Method

  • 期刊论文

  • 发表日期：

    2026年3月1日

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  • A. A. Bodunov,
  • A. V. Bochenkova

摘要

Abstract

The paper presents an implementation of the spin-unrestricted complete active space self-consistent field (UCASSCF) method. The key feature of this modification is the independent description of molecular orbitals for different spin projections: separate sets of spatial orbitals are used for α and β electrons, which provides a more flexible description of the multielectron wave function when taking into account the effects of spin polarization. Particular attention is paid to the analysis of the calculated expectation values of the spin-squared operator and their deviations from the exact values for a given multiplet state. The method was tested by calculating the potential energy curve of the ground state of the CN radical. It is shown that, in contrast to the UHF and UCASCI methods, spin contamination of the UCASSCF wave function is negligible in the region of the equilibrium geometry configuration of the radical. In this case, the total energy turns out to be systematically lower than that of the restricted version and the energy contribution from spin polarization is 0.3 eV. The development of unrestricted multiconfiguration quantum chemistry methods is essential for modeling the electronic structure of systems with significant spin polarization, including atoms and molecules in strong magnetic fields.