Electron correlation effects in cobalt fluorides CoF_n

Abstract

The molecular cobalt fluorides CoF 2 , CoF 3 and CoF 4 are studied and compared by employing different basis sets as well as Quantum Information Theory (QIT) to investigate their correlation effects. These prototypical monomers may be systematically extended in size yielding a novel quasi 1-dimensional, strongly correlated model system consisting of cobalt atoms bridged by oxygen atoms and fluorine termination on both ends. Accurate correlation energies are obtained using Full Configuration Interaction (FCI) and Full Configuration Interaction Quantum Monte Carlo (FCIQMC) calculations and the results are compared to Coupled Cluster and Density Matrix Renormalization Group (DMRG) energies. The analysis indicates the cobalt atom requires a larger number of one-electron basis functions than fluorine and the use of localized molecular orbitals may facilitate calculations for the extended systems. K E Y W O R D S correlation effects, electronic structure, orbital entanglement, strong correlation | INTRODUCTIONAccurate treatment of electron correlation is the main challenge in electronic structure theory. Electron correlation describes the difference between the Hartree-Fock (HF) solution, which represents a systematic approximation to solve the electronic Schrödinger equation, and the exact solution. HF relies on an ansatz for the wave function that is only exact for a system with noninteracting electrons, the Slater determinant. The family of wave function-based correlation methods provides an approach capable of resolving this error in the numerically exact limit of the employed one-electron basis set, known as the full configuration interaction (FCI) method. [1] Unfortunately it scales exponentially with the number of electrons and basis functions that limits practical application to very small systems with only a few electrons.One therefore tries to reduce the high computational cost by considering the largest contributions (electron configurations) only. A priori knowledge of which configurations to keep however is hard and only possible based on assumptions. As a consequence, a large number of different truncation schemes and methods have been developed, such as CCSD(T), [2,3] MRCI, [4] CASSCF, [4,5] DMRG, [6] FCIQMC, [7] and so on. An especially difficult class of problems are strongly correlated systems where partially occupied, near degenerate orbitals are present. This leads to multiple major configurations with similar weight and, due to the truncation of the FCI wave function, a balanced description of the orbitals with respect to all major configurations is required.Typical test systems to evaluate newly developed methods range from more abstract systems like the Hubbard model [8] over chains and lattices of hydrogen atoms [9,10] to small but realistic chemical systems like the dissociation of N 2 [7,11] or transition metal complexes. [12] We here propose a new system based on cobalt compounds that may be systematically extended in size. The prototypical "monomers" CoF 2 and CoF 4 (in its quadratic planar ...

Bandgaps in free‐standing monolayer TiO2: Ab initio diffusion quantum Monte Carlo study

“…As a promising tool, the QMC technique has been used previously to study diverse properties of bulk transition-metal oxides [23,[26][27][28], nanocrystals [29,30], atoms [31], molecules [32][33][34], 2D materials [35,36], diamond [37], MgSiO 3 perovskite [38], boron-nitride [39], point defects [40][41][42], Al [43], and Si [42,44,45] in previous studies. To date, to the best of our knowledge, this method has not been systematically used to calculate the bandgap parameters for freestanding 2D transition-metal oxides.…”