Dear Colleagues,

The electronic structure of molecules and solids determines their microscopic properties,  and the use of quantum many-body methods to unravel this structure has a long history. In recent years, however, novel techniques have been explored, and significant progress has been made in more traditional methods. After the success of the density matrix renormalization group in chemistry, the matrix product wave function ansatz has been generalized to various tensor network architectures, which offer the hope of a more effective description of the electron entanglement in molecules. In particular, the tree tensor structure has been analyzed, and methods to incorporate global spin symmetry have recently become available. Variants of coupled-cluster and projected Schrödinger equations techniques are being vigorously explored. Symmetry-breaking wave functions in conjunction with variation after projection have been studied. Another recent technique exploits a seniority-based hierarchy, of which the lowest rung are the fully paired geminal-based wave functions. Additionally, the direct variational determination of the two-particle density matrix under well-chosen N-representability conditions remains an actively pursued technique, and pure-state conditions (quantum marginal problem) have started to arouse interest. A Special Issue of Molecules will be devoted to the use of advanced quantum many-body techniques in electronic structure theory. Researchers who are involved in these fields are kindly invited to contribute to this Issue by submitting original research papers or reviews of their work. I think it is timely to gather some of the exciting new results and thereby make further advances in this field.

Dr. Dimitri Van Neck
Guest Editor

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• Quantum many-body methods
• Electronic structure
• Strongly correlated systems
• Tensor networks
• Symmetry breaking and restoration
• Coupled cluster
• Projected Schrödinger equation
• Geminals
• N-representability