Nonadiabatic coupling (NAC) is the most important physical quantity governing transitions between electronic states, such as internal conversion and radiationless relaxation.

The minimum-energy crossing points (MECPs) in the seam space where multiple electronic states are degenerate are the most important points which connect photochemical reaction pathways.

NACs can be computed with standard CIS and TDDFT methods, as well as with the spin-flip (SF) CIS and TDDFT variants, which afford correct topology around conical intersections.

NACs can also be computed with EOM-EE/IP/EA-CCSD methods.

Multiple algorithms for MECP location are available, including those that do not require NACs.