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Title: Perspective on Kramers symmetry breaking and restoration in relativistic electronic structure methods for open-shell systems

Journal Article · · Journal of Chemical Physics
DOI:https://doi.org/10.1063/5.0015279· OSTI ID:1714358
ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]
  1. Univ. of Washington, Seattle, WA (United States)
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Without rigorously symmetry constraints, solutions to approximate electronic structure methods may artificially break symmetry. In the case of relativistic electronic structure, if time-reversal symmetry is not enforced in calculations of molecules not subject to a magnetic field, it is possible to artificially break Kramers degeneracy in open shell systems. This leads to a description of excited states that may be qualitatively incorrect. Despite this, different electronic structure methods to incorporate correlation and excited states can partially restore Kramers degeneracy from a broken symmetry solution. For single-reference techniques, the inclusion of double and possibly triple excitations in the ground state provides much of the needed correction. Formally, however, this imbalanced treatment of the Kramers-paired spaces is a multi-reference problem, and so methods such as complete-active-space methods perform much better at recovering much of the correct symmetry by state averaging. Using multi-reference configuration interaction, any additional corrections can be obtained as the solution approaches the full configuration interaction limit. A recently proposed "Kramers' contamination" value is also used to assess the magnitude of symmetry breaking.

Research Organization:
Univ. of Washington, Seattle, WA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES); National Science Foundation (NSF)
Grant/Contract Number:
SC0006863; SC0021100; CHE-1856210; OAC-1663636
OSTI ID:
1714358
Alternate ID(s):
OSTI ID: 1657630; OSTI ID: 1870637
Journal Information:
Journal of Chemical Physics, Vol. 153, Issue 9; ISSN 0021-9606
Publisher:
American Institute of Physics (AIP)Copyright Statement
Country of Publication:
United States
Language:
English

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74 ATOMIC AND MOLECULAR PHYSICS
complete-active space self-consistent field
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full configuration interaction
spin-orbit interactions
degenerate energy levels
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Kramers theorem
multi-reference configuration interaction
particle symmetry
field theory
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS