The insert of negative element an(안)- is related to independence of verb in compound verb. In the compound verb, negative element an- can modify verb which has morphological and semantical independence.
The condition of root-separation is not same as the condition of insert of negative element. The insert of negative element an- can control root-separation but converse is not always true.
Specially the independence of verb in compound verb influences the insert of negative element. The semantic opacity of root obstructs root-separation and the insert of negative element in common. If root and verb are fused in meaning, the negative element can not be inserted in compound verb