Dempsey And Jimmy Make Peace
The Irishman, who was brought in to replace Steve Tosh at the club, has decided on the move due to the frustration at not getting a regular starting place inn recent months.
The English League One side appear able to offer Dempo first-team football as well as a salary that is probably outwith Aberdeen's current means for a squad player.
Calderwood told the Evening Express that he doesn't bear a grudge with Dempsey, who initially signed as an amateur to secure a move to the Dons.
He saud: "As much as we wanted to keep him, I accept that Gary has to look after himself.
"We simply aren't in the position to make an offer to him at this stage."