This chapter describes the calculation of properties depending on magnetic fields, both as created by an external magnetic field as well as the magnetic field created by a nuclear magnetic moment. This includes the two contributions to the ordinary spin-Hamiltonian used in NMR, nuclear shieldings and indirect nuclear spin-spin couplings constants. We also describe the calculation of the magnetic analogue of the polarizability, the molecular magnetizability . This property is of importance in NMR experiments where the reference substance is placed in another tube than the sample. We also shortly describe two properties very closely related to the magnetizability and nuclear shieldings respectively, the rotational g factor and the nuclear spin-rotation constants .
Three properties that in principle depend on the nuclear magnetic
moments are not treated here, namely the properties associated with
optical activity or, more precisely, with circular dichroism. These
properties are Vibrational Circular Dichroism
(VCD) , Raman Optical
activity (ROA) and Electronic Circular
Dichroism (ECD) will be
treated in Chapter 
.
Gauge-origin independent nuclear shieldings, magnetizabilities and rotational g tensors are obtained through the use of London atomic orbitals, and the theory is presented in several references [29, 30, 31, 32]. These properties are easy to calculate (from the user's point of view), and is thus given only a very brief description here.
The indirect spin-spin couplings are calculated by using the triplet linear response function , as described in Ref. [33]. These are in principle equally simple to calculate with DALTON as nuclear shieldings and magnetizabilities. However, there are 13 contributions to the spin-spin coupling constant from each nucleus. Furthermore, the spin-spin coupling constants put severe requirements on the quality of the basis set as well as a proper treatment of correlation, making the evaluation of spin-spin coupling constants a time consuming task. Some notes about how this time can be reduced is given below.