These terms are referring respectively to the electron and nucleus Zeeman interaction with the external magnetic field, and the hyperfine interaction. S and I are the electron and nuclear spin operators in the Zeeman basis (spin considered for simplicity), "ωe" and "ω"n are the electron and nuclear Larmor frequencies, and "A" and "B" are the secular and pseudo-secular parts of the hyperfine interaction. For simplicity we will only consider the case of |"A"|,|"B"|Â«|"ω"n|. In such a case "A" has little effect on the evolution of the spin system. During DNP a MW irradiation is applied at a frequency "ω"MW and intensity "ω"1, resulting in a rotating frame Hamiltonian given by