Approximation	Approximation description	Parameters
'IK−0'	Includes all product states between up to (and including) bas.level spins located anywhere within the system. For example, setting bas.level=5 would generate the basis that contains all spin correlations that involve five spins or fewer. The location of those spins is not taken into account.	bas.level
'IK−1'	Includes all product states between up to (and including) bas.level directly coupled spins and up to bas.space_level between spins that are closer together than the proximity cut‐off radius.   This basis starts from IK−0, but then also drops correlations between very remote spins — if a pair of spins is not coupled in any way, even the two‐spin order between them is not actually needed. Here, bas.level controls the maximum correlation order for spins connected by couplings, and bas.space_level controls the maximum correlation order for spins that are within the proximity cut‐off radius.	bas.level, bas.space_level
'IK−2'	Includes, for every spin, all correlations with all directly coupled spins and correlations with up to (and including) bas.space_level with spins that are closer together than the user‐specified proximity cut‐off.   This basis is similar to IK−1, except the truncation level around each spin is automatically set to the number of its direct coupling neighbours. This basis set can be quite large, but it is also very accurate.	bas.space_level