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Genoni A, Macetti G, Franchini D, Pieraccini S, Sironi M. X-ray constrained spin-coupled technique: theoretical details and further assessment of the method. ACTA CRYSTALLOGRAPHICA A-FOUNDATION AND ADVANCES 2019; 75:778-797. [PMID: 31692454 DOI: 10.1107/s2053273319011021] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/24/2019] [Accepted: 08/07/2019] [Indexed: 11/11/2022]
Abstract
One of the well-established methods of modern quantum crystallography is undoubtedly the X-ray constrained wavefunction (XCW) approach, a technique that enables the determination of wavefunctions which not only minimize the energy of the system under examination, but also reproduce experimental X-ray diffraction data within the limit of the experimental errors. Initially proposed in the framework of the Hartree–Fock method, the strategy has been gradually extended to other techniques of quantum chemistry, but always remaining limited to a single-determinant ansatz for the wavefunction to extract. This limitation has been recently overcome through the development of the novel X-ray constrained spin-coupled (XCSC) approach [Genoni et al. (2018). Chem. Eur. J.
24, 15507–15511] which merges the XCW philosophy with the traditional spin-coupled strategy of valence bond theory. The main advantage of this new technique is the possibility of extracting traditional chemical descriptors (e.g. resonance structure weights) compatible with the experimental diffraction measurements, without the need to introduce information a priori or perform analyses a posteriori. This paper provides a detailed theoretical derivation of the fundamental equations at the basis of the XCSC method and also introduces a further advancement of its original version, mainly consisting in the use of molecular orbitals resulting from XCW calculations at the Hartree–Fock level to describe the inactive electrons in the XCSC computations. Furthermore, extensive test calculations, which have been performed by exploiting high-resolution X-ray diffraction data for salicylic acid and by adopting different basis sets, are presented and discussed. The computational tests have shown that the new technique does not suffer from particular convergence problems. Moreover, all the XCSC calculations provided resonance structure weights, spin-coupled orbitals and global electron densities slightly different from those resulting from the corresponding unconstrained computations. These discrepancies can be ascribed to the capability of the novel strategy to capture the information intrinsically contained in the experimental data used as external constraints.
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Genoni A, Franchini D, Pieraccini S, Sironi M. X‐ray Constrained Spin‐Coupled Wavefunction: a New Tool to Extract Chemical Information from X‐ray Diffraction Data. Chemistry 2018; 24:15507-15511. [DOI: 10.1002/chem.201803988] [Citation(s) in RCA: 16] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/03/2018] [Indexed: 11/08/2022]
Affiliation(s)
- Alessandro Genoni
- Université de Lorraine CNRS, Laboratoire LPCT 1 Boulevard Arago 57078 Metz France
| | - Davide Franchini
- Dipartimento di Chimica Università degli Studi di Milano Via Golgi 19 20133 Milano Italy
| | - Stefano Pieraccini
- Dipartimento di Chimica Università degli Studi di Milano Via Golgi 19 20133 Milano Italy
- Istituto di Scienze e Tecnologie Molecolari (ISTM), CNR Via Golgi 19 20133 Milano Italy
- Consorzio Interuniversitario Nazionale per la, Scienza e Tecnologia dei Materiali (INSTM), UdR Milano Via Golgi 19 20133 Milano Italy
| | - Maurizio Sironi
- Dipartimento di Chimica Università degli Studi di Milano Via Golgi 19 20133 Milano Italy
- Istituto di Scienze e Tecnologie Molecolari (ISTM), CNR Via Golgi 19 20133 Milano Italy
- Consorzio Interuniversitario Nazionale per la, Scienza e Tecnologia dei Materiali (INSTM), UdR Milano Via Golgi 19 20133 Milano Italy
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