Causes of failure to achieve an SCF

In a system where a biradical can form, such as ethane decomposing into two CH3 units, the normal RHF procedure can fail to go self-consistent. If the system has marked biradicaloid character, then BIRADICAL or UHF and TRIPLET can often prove successful. These options rely on the assumption that two unpaired electrons can represent the open shell part of the wave-function.