e ion 11 or e ion 12 involves summation running over all the atoms (k) in a cell, e is the charge of an electron, and A is the area of the cell for sake of 2D unit. The response of the k-th atom's internal coordinate along the a-direction (u 1 (k)) in response to a macroscopic strain (η 1 ) in the same direction is measured by du1(k) dη1 . Similarly, ) part of the total piezoelectric stress constant e11and e12 in 2D piezoelectric unit of 10 -10 C/m of the MOX2 monolayers , Born eective charge Z11of metal (Ti and V), O, and halogen X= F, Cl, and Br in charge of an electron (|e|) unit. du 1 dη 1 or du 1 dη 2 represent the change of the atomic coordinates along a-direction in response to a strain along a-direction (η1) or b-direction (η2), respectively. represents the change in the k-th atom's internal coordinate along the a-direction (u 1 (k)) in response to a macroscopic strain (η 2 ) along b-direction. Relaxing the atomic positions in response to the strain η 1 and η 2 , we obtain the slope du1(k) dη1 and du1(k) dη2 , respectively. We notice that large e ion 11 of TiOCl 2 and TiOBr 2 monolayers comes from their large Z 11 (see Table ) and du1 dη1 (see Table ). Also, we see that du1 dη1 of Ti/V/O is an order of magnitude larger than du1 dη2 i.e., the uniaxial strain η 1 can displace atoms along a-direction more than the η 2 . This also results large dierence between e ion 11 and e ion 12 . We observe that AFM1 order of VOCl 2 and VOBr 2 marginally enhances e 11 (see Table ). Note that e 11 of MOX 2 monolayers is signicantly (about 610 times) larger than that of well-known 1H-type piezoelectric monolayers e.g., 1H-MoS 2 [4, 6, 8]. We notice that e ion 11 of MOX 2 monolayers is an order of magnitude larger than that of 1H-MoS 2 or 1H-VS 2 , although their electronic part is quite comparable . Because both Z 11 and du1 dη1 of MOX 2 monolayers are remarkably higher than those of 1H-MoS 2 or 1H-VS 2 . Our e 11 is quite comparable with that of group-IV monochalcogenide monolayers such as SnS , although dierence between e 11 and e 12 in group-IV monochalcogenides is not as pronounce as in MOX 2 monolayers.