Single-Reference Methods Based on Complex Orbital in Electronic Structure Calculations for High-Symmetry Systems

In nonrelativistic and scalar-relativistic electronic structure calculations, molecular orbitals (MOs) are usually chosen as real functions and multideterminant wave functions are required to describe multireference (MR) states. However, certain specific MR states in atoms, linear molecules, and nonlinear molecules possessing real two-dimensional irreducible representations can be represented by a single determinant when complex MOs are employed. For atoms and linear molecules, MOs that are eigenfunctions of the angular momentum operator are used, and the resulting single-determinants states are labeled by the angular momentum quantum numbers. Within density functional theory (DFT), an angular-momentum symmetry-broken method, analogous to the spin-symmetry broken method, is also developed for selected MR states in atoms and linear molecules. The performance of MP2, CCSD, CCSD(T), and DFT using complex MOs is assessed for low-spin states of some p- and d-block atoms, diatomic and nonlinear molecules, and for high-spin states of selected transition-metal diatomic molecules. CCSD(T) with complex MOs generally yields highly accurate results when applicable, while DFT provides reasonable accuracy with appropriately chosen exchange-correlation functionals.