Equation-of-motion coupled cluster method for the description of the high spin excited states

Abstract

The equation-of-motion (EOM) coupled cluster (CC) approach in the version applicable for the excitation energy(EE) calculations has been formulated for high spin components. The EE-EOM-CC scheme based on the restricted Hartree-Fock reference and standard amplitude equations as used in the Davidson diagonalization procedure yields the singlet states. The triplet and higher spin components require separate amplitude equations. In the case of quintets, the relevant equations are much simpler and easier to solve. Out of 26 diagrammatic terms contributing to the R1 and R2 singlet equations in the case of quintets, only R2 operator survives with 5 diagrammatic terms present. In addition all terms engaging three body elements of the similarity transformed Hamiltonian disappear. This indicates a substantial simplification of the theory. The implemented method has been applied to the pilot study of the excited states of the C2 molecule and quintet states of C and Si atoms.

Relativistic calculations of AuSi+ and AuSi−

“…For many years, research has been conducted on methods that would allow an accurate description of the excited states of molecules. These are studies on single reference methods, for example, such as coupled cluster theory (CC), equation of motion (EOM), time‐dependent density functional theory, or multireference (MR) methods such as MR configuration interaction (MRCI), MR coupled cluster (MRCC, FS‐MRCC), MR perturbation theory, MR density functional theory and much more (see the review). This only shows how complicated is the problem to study the excited states of molecules.…”

Theoretical studies of the ground and excited states of the NaLi molecule by the IOTC CASPT2 method

“…For many years, research has been conducted on methods that would allow an accurate description of the excited states of molecules. These are studies on single reference methods, for example, such as coupled-cluster theory (CC) [3], Maria Barysz teomjb@chem.umk.pl 1 Department of Quantum Chemistry, Faculty of Chemistry, Nicolaus Copernicus University, 7-Gagarina St., 87-100, Toruń, Poland equation of motion (EOM) [4][5][6][7], time-dependent density functional theory (TDDFT) [8,9], or multireference (MR) methods such as MR configuration interaction (MRCI) [10], MR coupled-cluster (MRCC, FS-MRCC) [11,12], MR perturbation theory (MRPT), MR density functional theory (MRDFT), and much more (see the review [12]). This only shows how complicated the problem is.…”