Changing the number of unpaired electrons in a molecule directly affects its magnetic properties. Molecules with unpaired electrons exhibit paramagnetic behavior, while those with all paired electrons are diamagnetic.Paramagnetic molecules have unpaired electrons, which create a net magnetic moment. In the presence of an external magnetic field, these molecules align with the field, resulting in a positive magnetic susceptibility. The more unpaired electrons a molecule has, the stronger its paramagnetic properties.Diamagnetic molecules, on the other hand, have all their electrons paired. They have no net magnetic moment and are not attracted to a magnetic field. In fact, they exhibit a weak repulsion to an external magnetic field, resulting in a negative magnetic susceptibility.Quantum chemistry methods can be used to calculate the magnetic properties of molecules by determining the number of unpaired electrons and their distribution in the molecule. Some common quantum chemistry methods used for this purpose include:1. Hartree-Fock HF method: This is a mean-field approach that approximates the electronic wave function of a molecule by considering each electron to be moving in an average field created by all other electrons. It provides a good starting point for understanding the electronic structure and magnetic properties of a molecule.2. Density Functional Theory DFT : This method is based on the electron density of a molecule rather than its wave function. It is more accurate than the HF method and can be used to study larger systems. DFT can provide information about the distribution of unpaired electrons and their contribution to the magnetic properties of a molecule.3. Post-Hartree-Fock methods: These methods, such as Configuration Interaction CI and Coupled Cluster CC theory, account for electron correlation effects that are not considered in the HF method. They provide more accurate results for the magnetic properties of molecules, especially those with strong electron correlation effects.4. Multi-reference methods: For molecules with complex electronic structures, such as those with multiple unpaired electrons or near-degeneracy, multi-reference methods like Complete Active Space Self-Consistent Field CASSCF and Multi-Configuration Pair-Density Functional Theory MC-PDFT can provide more accurate descriptions of their magnetic properties.By using these quantum chemistry methods, one can calculate the number of unpaired electrons, their distribution in the molecule, and the resulting magnetic properties, such as magnetic susceptibility and magnetic moments. These calculations can help in understanding the behavior of molecules in the presence of magnetic fields and can be useful in designing materials with specific magnetic properties.