7.9 Correlated Excited State Methods: The ADC(n) Family

7.9.5 Spin-Flip ADC Methods

The spin-flip (SF) method497, 498, 499, 552 is used for molecular systems with few-reference wave functions like diradicals, bond-breaking, rotations around single bonds, and conical intersections. Starting from a triplet (ms=1) ground state reference a spin-flip excitation operator {C^J}={caβciα; caβcbσciαcjσ, a <  b, i <  j} is introduced, which flipped the spin of one electron while singlet and (ms=0) triplet excited target states are yielded. The spin-flip method is implemented for the ADC(2) (strict and extended) and the ADC(3) methods.552 Note that high-spin (ms=1) triplet states can be calculated with the SF-ADC method as well using a closed-shell singlet reference state. The number of spin-flip states that shall be calculated is controlled with the $rem variable SF_STATES.