A Combined Spin-Flip and IP/EA Approach for Handling Spin and Spatial Degeneracies: Application to Double Exchange Systems

Abstract

<div>Many multiconfigurational systems, such as single-molecule magnets, are difficult to study using traditional computational methods due to the simultaneous existence of both spin and spatial degeneracies. In this work, a new approach termed n-spin-flip Ionization Potential/Electron Affinity (<i>n</i>SF-IP or <i>n</i>SF-EA) is introduced which combines the spin-flip method of Anna Krylov with particle-number changing IP/EA methods. We demonstrate the efficacy of the approach by applying it to the strongly-correlated N<sub>2</sub><sup>+</sup> as well as several double exchange systems. We also demonstrate that when these systems are well-described by a double exchange model Hamiltonian, only 1SF-IP/EA is required to extract the double exchange parameters and accurately predict energies for the low-spin states. This significantly reduces the computational effort for studying such systems. The effects of including additional excitations (using a RAS-<i>n</i>SF-IP/EA scheme) are also examined, with particular emphasis on hole and particle excitations.</div>

Restricted active space configuration interaction methods for strong correlation: Recent developments

“…The RASCI( h , p ) approach has been implemented in the Q‐Chem program 59 following an integral driven algorithm 60 mixed with configurational driven strategies 50,61–63 for the hole and particle contributions 42 . Later on, various forms of the RASCI method 64 were also implemented in a pilot code within the Psi4 program 65 . In the Q‐Chem implementation, the computational code runs over nonzero one‐ and two‐electron interaction terms involving orbitals within the RAS2 space, and identifies pairs of configurations coupled through each term of the Hamiltonian.…”

Toward multifunctional molecular cells for quantum cellular automata: exploitation of interconnected charge and spin degrees of freedom

“…It is worth noting that we restrict our consideration by the case of non-degenerate orbitals (see, for example, ref. 57 which deals with more general case when the orbital (special) degeneracy is involved).…”

“…[54][55][56] . In recent years significant progress has been achieved in the quantitative evaluation of the effect of the DE with the aid of advanced quantum-chemical methods (see 57 and refs. therein).…”