Atom-centred density matrix

Rega et al. [104] have proposed the QM/MM approach ADMP/ONIOM which implements the atom-centred density matrix propagation within the QM/MM ONIOM scheme of Maseras and Morokuma [105]. The ONIOM approach consists of separating the system in parts described at different levels of theory, for example a model part treated at Hartree-Fock or DFT level (QM), embedded in surroundings that will be treated at a molecular mechanics level (MM). The real system is composed of the model system and of the surrounding system, and the ONIOM energy is ... 

The electrostatic interactions in a molecule are determined by the structure of the density matrix D. In constructing D from an atomic orbital or atomic spinor basis, we incorporate a lot of redundant information in the Gaussian overlap charge distributions, since most of the electron density is concentrated near the nuclei. One should therefore try to transform D into a block diagonal form in which each dense block corresponds to a one-centre density so that the... 

The carbenes obtained by photolysis of a- and /S-naphthyldiazo-methanes each show two triplet signals in their EPR spectra (Trozzolo et al., 1965). The observed spectra are independent of the matrix used, and have therefore been assigned to geometric isomers of the carbenes (10 a, b 11 a, b). Arylmethylenes are expected to show such behaviour provided that the lifetime of the individual isomeric forms is long on the EPR time-scale and provided that the spin densities on nearby atoms in the w-system, particularly the carbon atoms ortho to the divalent centre, are different. This leads to different interactions between the... 

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