LVACPOTAV

This tag switches on the computation of the vacuum potential; i.e. the average of the local potential in the vacuum region.

The vacuum potential is one of the quantities needed to compute the workfunction (see here for a step-by-step guide on how to compute the workfunction with VASP). The vacuum potentials are written to the OUTCAR file and can be accessed with the following command

grep upper OUTCAR

which will produce the vacuum potentials. Note that two vacuum potentials will be produced, one corresponding to the upper termination of the slab and one corresponding to the lower. Depending on the system you are studying, one might be more interesting than another.