The project will develop a coupled-cluster program for the calculation of molecular magnetizabilities and rotational g tensors. Gauge origin independence and fast basis set convergence will be achieved through the use of London atomic orbitals. Our goal i s to develop a code for calculation of magnetic properties using coupled-cluster singles and doubles wave functions, including also a perturbative treatment of triples corrections, enabling benchmark results for molecular magnetizabilties and rotational g tensors to be obtained. From microwave Zeeman experiments, highly accurate results in the gas phase can in principle be obtained for second-order magnetic properties such as magnetizability anisotropies and rotational g tensors. These results will provide us with important bench mark results with which to compare our calculations. However, approximations and assumptions of different kinds are often used to extrapolate measured results to other isotopic species of the molecule. We will, using the new code, explore the validity of these approximations, in particular in relation to mo lecular vibrations and isotopic substitution.