Pal.gr.108

chrē [Baskets] and things for adornment [they use]. The lavish man uses [them]. And also the one being lavish. Chrēnai means to exist. And it is. Which words are written with the ei diphthong? Chrēsthai is that which occurs in life; for example, chreia [usage/need] and to chreon [the necessary/debt], from the neuter chrēn. As in Sophocles: "one ought (chrēn) to say what is useful." That is, it was useful to speak. From these is also composed to chreōn [fate/necessity]. There is also the indeclinable chrea. And chreōn is the qualitative of the same; a certain chreōn. Chreōn is something "more"; it comes from chreon by the pleonasm of the ei diphthong, as is accustomed to happen to those [words] that expand the ei diphthong in nouns and verbs. For example, adelphos [becomes] adelpheios; polemisō, polemeisō; thaumasō, thaumeisō. And it is: "God, how great a glory this is!" Theos is a noun, but theein is to run. For the Greeks called the bodies [moving] through the ō "gods," whirling in the ether. We have taken the name from the middle. Then, they were indicated [?] from theō [I run]. But those of the e [stems] with a fixed power to receive contraction in the second and third person are indicated through the ei. For example, cheeis [becomes] cheis; cheei, chei. Rheeis, rheis; rheei, rhei. Zeeis, zeis; zeei, zei. Pleeei [means] the traveling upon the sea from the [verb in] ō; pleeis, pleis; pleeei, plei. Cheei is the "more" from the [verb in] ō, or "I dive" [kolymbō]. And it has also become ei.
ei But those unable to receive contraction in the second and third person are written with the ai diphthong.
aiō For example, palaiō [I wrestle]; lalaomai [?] meaning "I stir the spirit," and also "I strike." Aroueis [from] palaiō meaning "I plow." And knaiō meaning "I scrape away." Ptaiō [meaning] "fail/omit." And sympaiō meaning "I strike together." Ptaiō, klaiō [I weep], kaiō [I burn].