Changes in the electronic structure of a molecule can significantly affect its magnetic properties. The presence or absence of unpaired electrons in a molecule plays a crucial role in determining its magnetic susceptibility. Magnetic susceptibility is a measure of how much a material will become magnetized in an external magnetic field.Molecules with unpaired electrons are generally paramagnetic, meaning they are attracted to an external magnetic field. This is because the unpaired electrons have a magnetic moment due to their spin and orbital angular momentum. When an external magnetic field is applied, these magnetic moments tend to align with the field, causing an attraction. The more unpaired electrons a molecule has, the greater its magnetic susceptibility.On the other hand, molecules with no unpaired electrons are diamagnetic, meaning they are weakly repelled by an external magnetic field. This is because all the electrons are paired, and their magnetic moments cancel each other out. In a diamagnetic molecule, the applied magnetic field induces a small opposing magnetic field, resulting in a weak repulsion.Quantum chemistry calculations can be used to predict the magnetic properties of molecules by determining their electronic structure. One of the most common methods for doing this is called the Hartree-Fock method, which is a self-consistent field method that approximates the electronic wavefunction of a molecule. Other more accurate methods include configuration interaction CI , coupled cluster CC , and density functional theory DFT .These theoretical models can be used to calculate the molecular orbitals and their corresponding electron configurations. By analyzing the electron configurations, one can determine the presence or absence of unpaired electrons in the molecule and, consequently, predict its magnetic susceptibility.For example, if a molecule has an open-shell electronic configuration i.e., unpaired electrons , it will likely be paramagnetic and have a positive magnetic susceptibility. Conversely, if a molecule has a closed-shell electronic configuration i.e., no unpaired electrons , it will likely be diamagnetic and have a negative magnetic susceptibility.In summary, changes in the electronic structure of a molecule, specifically the presence or absence of unpaired electrons, can significantly affect its magnetic properties. Quantum chemistry calculations, such as Hartree-Fock, CI, CC, and DFT, can be used to predict the electronic structure of molecules and, in turn, their magnetic susceptibility.