where ℎ 𝑝𝑞 represents the one-electron integral, 𝑔 𝑝𝑞𝑟𝑠 denotes the two-electron integral, and 𝐸 ̂𝑝𝑞 is the excitation operator for the electronic molecular orbitals between p and q. In the four-component calculations, positronic molecular orbitals are generally disregarded in Eq. ( ) under the no-pair approximation. Relativistic effects can be incorporated through one-electron integrals evaluated using a relativistic one-electron operator defined in the respective theories, such as the fourcomponent or the X2C Hamiltonian. The Coulomb interaction is often used as the two-electron operator for evaluating two-electron integrals both in non-relativistic and relativistic cases. In spindependent relativistic cases, the labels p, q, r, and s run over molecular spin orbitals (or molecular spinors) instead of molecular spatial orbitals because spin symmetry is no longer applicable.