We present an extension of our semiempirical floating occupation MO-CI approach for the determination of ground and excited state potential energy surfaces of interest in photochemistry. The QM/MM variant of the method, which allows for electrostatic and van der Waals interactions between the QM and MM subsystems, is supplemented with a treatment of covalent interactions based on Antes and Thiel’s connection atom approach. We concentrate on the correct treatment of electrostatic interactions concerning the connection atom, on the specific requirements for the representation of excited states, and on the transferability of the optimal parameters. We show the viability of the method with four examples of connection atoms: S in a thioether bridge, acylic C, aliphatic C, and N in a peptide. The results obtained with the QM/MM treatment compare well with all-QM results of the same level.
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This paper is dedicated to our friend and teacher Jacopo Tomasi, a pioneer of the elaboration of electrostatics and QM/MM concepts and methods. Our research work owes a lot to Jacopo: inspiration, warnings, criticism and, sometimes, his most welcome appreciation.
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Toniolo, A., Ciminelli, C., Granucci, G. et al. QM/MM connection atoms for the multistate treatment of organic and biological molecules. Theor Chem Acc 111, 270–279 (2004). https://doi.org/10.1007/s00214-003-0522-7