This provides a convenient and straightforward approach, due to the extensive, thoroughly tested, and commercially available methods developed in the electronic structure community to solve Eq. 2 in an accurate and efficient fashion. This approach does not require any redevelopment of new electronic-photonic structure theory but rather utilizing what is already available for a new purpose. More details of using this approach to compute the excited state properties are described in the Theoretical Details together with brief computational details at the end of this letter as well as in the Supporting Information.