Calculation of magnetic properties. VI. Electron correlated nuclear shielding constants and magnetizabilities for thirteen small molecules

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Abstract
The theory of relaxed density matrices has been developed for the calculation of second-order response properties at third-order Mo/ller–Plesset (MP3) and linearized coupled cluster double excitation (L-CCD) levels of theory. The ensuing algorithm is applied to the determination of the isotropic and anisotropic nuclear magnetic shielding constants and magnetizabilities for thirteen molecules (H2, N2, F2, HF, CO, HCN, HNC, H2O, NH3, H2O2, HCHO, CH4, and HCCH). The method uses conventional gauge-variant orbitals but, by using large basis sets, produces results which are equivalent to those found with gauge-including orbitals. In general the L-CCD values of the magnetizabilities are consistent with those obtained with multiconfigurational self-consistent-field (MCSCF) methods for molecules which have been treated by this method. For the nuclear shieldings, when there is a difference between L-CCD and MP3, the former gives results closer to the coupled-cluster singles and doubles level treatment augmented with a perturbation correction for connected triple excitations [CCSD(T)] which is our reference point. Again the results for the shieldings at the L-CCD level are quite good. We also use the paramagnetic components of the shieldings and magnetizabilities to determine the spin-rotation constants and rotational g tensors, respectively. These quantities are important since they may be compared more directly with experiment than the magnetizabilities and shieldings.