TestIfDETinCASBit Function

    logical function TestIfDETinCASBit(ilutnI)

        ! In:
        !    iLutNI: bit string representation of a determinant.
        ! Returns:
        !    true if the determinant is in the complete active space.

        integer(n_int), intent(in) :: iLutnI(0:NIfD)

        ! A determinant is in the CAS iff
        !  a) all orbitals in the core space are occupied;
        !  b) no orbitals in the external space are occupied;
        ! Thus ANDing the determinant with CASMask (containing set bits for
        ! the core and external orbitals) will give precisely the core
        ! orbitals if the determinant is in the CAS.

        TestifDETinCASBit = all(iand(iLutNI, CASMask) == CoreMask)

    end function TestIfDETinCASBit