Multi-configuration quasi-degenerate perturbation theory

Multi-Configurational Quasi-Degenerate Second-Order Perturbation Theory... 

DIM = diatomics-in-molecules MRDCI = multi-reference single and double excitation configuration interaction QDPT = quasi-degenerate perturbation theory. 

The methods of choice must be adequate for manifolds of electronic states that are localized around a lanthanide ion in a solid host. The combination of a solid environment, a heavy element, and 4/, 5d, and other open-shells, demands the consideration of the effects of the solid host, the use of relativistic Hamiltonians up to spin-orbit coupling, the correct treatment of static and dynamic correlation, and handling large manifolds of quasi-degenerate excited states. We decided to use embedded-cluster wavefunction theory-based (EC-WFT) methods, with a two-component relativistic Hamiltonian to be used in two-steps, a multi-configurational variational treatment of static correlation, and a multireference second-order perturbation theory treatment of dynamic correlation. 

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