The rule is that the accent may fall on the antepenult if the ultima is short. While αι is long it does sometime count as short so I assume that is what is happening here.
EDIT
A final αι always counts as short unless it is opatitive.
The modern Greeks use a stress accent, and they stress the entire syllable. There is no distinction between the circumflex or the acute. This is certainly the easiest way for a foreign language speaker to accent Greek.
At some time in the past the Greek language switched from a pitch to stress accent and lost vowel length. It is generally thought that these two phenomenon occurred at the same time, and that may be a reasonable guess, although pitch accents seem to come and go in various language groups for all sorts of reasons.
If you are going to speak ancient Greek aloud, the most important thing by far is getting vowel length correct – this is my opinion – and you can still use a stress accent. Just accent the entire syllable.
If you decide to make the multi-year and likely futile journey involved in attempting a pitch accent along with vowel length (and you can check out my YouTube channel for where I’m at on in after 2 years), then yes, the pitch varies depending on what part of the diphthong you are at. You have to break up the word by mora.
Won’t the accent be the same as with a long monophthong? It’s a single syllable, after all.
It’s the same as for a monophthong, yes. But the point about syllables isn’t actually persuasive. The syllable is the basic sound unit of modern Greek or English, but the basic unit of ancient Greek was the mora – according to Allen and Sommerstein and others. Both long vowels and diphthongs are two mora – like in modern Japanese.
(It’s just a modern spelling convention to place the accent on the second vowel.)
If I were going to argue against a modern convention, it would be the convention of making a circumflex a single character in length. It should cover the entire diphthong, as it does in the manuscripts.
Given that a circumflex is a high pitch followed by a svarita on the same syllable (but in fact the next mora), placing the acute on the first vowel (mora) of the diphthong would be perverse.
That was my point, more or less, only I didn’t see the need to introduce chiasmus_a to new technical terms. I simply wanted to answer his question in a non-confusing way.
Plural of mora is morae, btw.