INCAR file:
IBRION=6
ISIF≥3
OUTPUT file:
SYMMETRIZED ELASTIC MODULI (kBar)
The elastic moduli for rigid ions are written after this line
ELASTIC MODULI CONTR FROM IONIC RELAXATION (kBar)
The ionic contributions
TOTAL ELASTIC MODULI (kBar)
The final elastic moduli including the contributions for distortions with rigid ions and from the ionic relaxations are written after this line
From: https://cms.mpi.univie.ac.at/wiki/index.php/IBRION
Other techniques: https://www.researchgate.net/post/How_VASP_calculate_elastic_constants