where Sµν = ⟨χµ | χν ⟩ is the overlap matrix involving displaced basis functions. We compute Oij twice: once between the HOMO of the isolated donor molecule (HOMO i ) and the HOMO of the donor molecule within the complex (HOMO c ), using ghost functions to ensure that both calculations have the same basis functions; and second, between the LUMO of the isolated acceptor molecule (LUMO i ) and the LUMO of the acceptor molecule within the complex (LUMO c ). We define η as the average of these two quantities: