I used DFPT to calculate the piezoresponse for barium titanate -- a simple 5 atom unit cell.

Calculation ran quickly and without incident.

BUT.... I get a negative value for d33 (-97). See the matrix below. The magnitude is correct. BUT the sign

is wrong: BTO is not a negative piezo mat'l.

Or is it the case that abinit reports the signs "oppositely"?

Piezoelectric d tensor (relaxed ion) (unit:pc/N)

-96.67275336 42.33753525 42.06348693

41.86155809 -97.07601604 42.22293861

41.73607024 41.79187521 -97.16557179

12.81189036 -10.81879086 -12.15171696

-10.65680069 12.87695345 -12.30689921

-10.70041572 -12.17815700 12.86743783