Excited States, Symmetry Breaking, and Unphysical Solutions in State-Specific CASSCF Theory

被引:15
作者
Marie, Antoine [1 ,2 ]
Burton, Hugh G. A. . [1 ]
机构
[1] Univ Oxford, Dept Chem, Phys & Theoret Chem Lab, Oxford OX1 3QZ, England
[2] Univ Toulouse, CNRS, UMR 5626, UPS,Lab Chim & Phys Quant, F-31062 Toulouse, France
关键词
FULL CONFIGURATION-INTERACTION; DENSITY-FUNCTIONAL THEORY; HARTREE-FOCK EQUATION; SPACE SCF METHOD; MOLECULAR-ORBITAL METHODS; COUPLED-CLUSTER METHODS; UNITARY-GROUP-APPROACH; NEWTON-RAPHSON; WAVE-FUNCTIONS; EXCITATION-ENERGIES;
D O I
10.1021/acs.jpca.3c00603
中图分类号
O64 [物理化学(理论化学)、化学物理学];
学科分类号
070304 ; 081704 ;
摘要
State-specific electronic structure theory provides a route toward balanced excited-state wave functions by exploiting higher-energy stationary points of the electronic energy. Multiconfigurational wave function approximations can describe both closed- and open-shell excited states and avoid the issues associated with state-averaged approaches. We investigate the existence of higher-energy solutions in complete active space self-consistent field (CASSCF) theory and characterize their topological properties. We demonstrate that state-specific approximations can provide accurate higherenergy excited states in H2 (6-31G) with more compact active spaces than would be required in a state-averaged formalism. We then elucidate the unphysical stationary points, demonstrating that they arise from redundant orbitals when the active space is too large or symmetry breaking when the active space is too small. Furthermore, we investigate the singlet-triplet crossing in CH2 (6-31G) and the avoided crossing in LiF (6-31G), revealing the severity of root flipping and demonstrating that state-specific solutions can behave quasi-diabatically or adiabatically. These results elucidate the complexity of the CASSCF energy landscape, highlighting the advantages and challenges of practical state-specific calculations.
引用
收藏
页码:4538 / 4552
页数:15
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