The determinant sign, is perhaps the trickiest part of computing CI-matrix elements. Fortunately, , clears this issue on page 2-5, and uses Γ to denote the sign-convention of a determinant. Because annihilating (denoted by a, target MO is occupied) and creation (denoted by a † , target MO is vacant) operations are mutually exclusive they may be done with-in the same MO: