Davidson and Clark’s equations for calculating the local spin of molecules were rewritten and simplified as matrices algebra. By applying our program, we calculated the local spin of O2 and NO molecules and obtained almost the same results as those by Davidson and Clark’s method. The local spin and the magnetic coupling constant J of the simple Van der Waals complex HHeH were calculated as well to investigate the relationship between the geometric structures and magnetic values. Davidson and Clark’s equations for calculating the local spin of molecules were rewritten and simplified as matrices algebra. By applying our program, we calculated the local spin of O2 and NO molecules and obtained almost the same results as those by Davidson and Clark’s method. The local spin and the magnetic coupling constant J of the simple Van der Waals complex HHeH were calculated as well to investigate the relationship between the geometric structures and magnetic values.