Decomposing the Hartree-Fock one-electron density matrix and a virtual pseudodensity matrix, we obtain an orthogonal set of normalized molecular orbitals with local character to be used in post-Hartree-Fock calculations. The applicability of the procedure is illustrated by calculating CCSD(T) energies and CCSD molecular properties in reduced active spaces.

Although for planar conjugated hydrocarbons the out-of-plane component of proton magnetic shielding is an unquestionable quantitative aromaticity indicator, the same is not true for tetraazanaphthalenes. As in these compounds the (core + sigma)-currents associated to the nitrogen nuclei diminish the perpendicular component of shielding, abnormal values of (1)H NMR sigma(zz) are obtained. Therefore, a consistent aromaticity measure must be based only on the pi-contribution to the out-of-plane component of proton magnetic shielding. Otherwise, the behavior of these compounds in presence of an external magnetic field parallels that of naphthalene, with the nitrogen nuclei contributing to the ring current in a comparable amount to carbon nuclei. The pi-current contribution to magnetic shielding represents 6-8% of the out-of-plane shielding for nitrogen and 9-12% for carbon.

The local response to an external magnetic field normal to the molecular plane of naphthalene and anthracene was investigated via current density and magnetic shielding density maps. The Biot-Savart law shows that the deshielding caused by pi-ring currents in naphthalene is stronger for alpha- than for beta-protons due to geometrical factors. The shielding tensor of the carbon nuclei in both molecules is strongly anisotropic and its out-of-plane component determines the up-field chemical shift of (13)C in nuclear magnetic resonance spectra. The pi-ring currents flowing beyond the C-skeleton in front of a probe carbon nucleus, and on remote parts of the molecular perimeter, yield positive contributions to the out-of-plane component of carbon shielding as big as approximately 10-15% of the total values. Near Hartree-Fock estimates of magnetizability and magnetic shielding at the nuclei fully consistent with the current model are reported.

Multiscale modeling and simulation (MMS) combining B97-D/TZV2P DFT calculations and molecular dynamics simulations are performed to investigate the adsorption of hydrogen over coronene as a model of graphene.

Different force fields for the graphene–CH4 system are proposed including pseudo-atom and full atomistic models.