Calculation of contact densities and Mössbauer isomer shifts utilising the Dirac-exact two-component normalised elimination of the small component (2c-NESC) method.

Utilizing the analytic derivative formalism for the Mössbauer isomer shift in connection with the Dirac-exact two-component Normalized Elimination of the Small Component (2c-NESC) method a new approach to the analytic calculation of the contact densities at the nuclei has been developed and implemented in the general purpose NESC programme. The new approach is applied to the calculation of the contact densities as well as contact density differences in several iodine-, gold-, and mercury-containing molecules. Substantial differences between the contact densities obtained by the spin-free 1c-NESC method and the 2c-NESC method are found, which demonstrate the importance of spin-orbit coupling. However, the influence of spin-orbit coupling on the contact density differences between the sample and the reference nuclei is found to be modest. This result suggests that a low-cost determination of accurate contact densities at the nuclei can be achieved by combining the 1c-NESC densities obtained at the correlated wavefunction level of theory with the contact density differences obtained at the 2c-NESC/DFT level. [ABSTRACT FROM AUTHOR]