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I am trying to run a calculation where I input a negative value in the POSCAR file in place of the lattice parameter to specify cell volume (as stated in the vasp manual). When I do this, the atoms are much too far apart. I found a similar case in the hands-on tutorial (IV) where they do this for fcc nickel, but even when I use the exact POSCAR they have, the atom spacing is much further than typical fcc Ni spacing. Has anyone encountered this problem? I have tried using both direct and cartesian coordinates with no change. Here is my POSCAR file:

Barium Oxide
-177.504
1 0 0
0 1 0
0 0 1
4 4
cart
0 0 0
0.5 0 0.5
0 0.5 0.5
0.5 0.5 0
0.5 0 0
0 0.5 0
0 0 0.5
0.5 0.5 0.5

The volume I'm using here is for a cubic cell with a lattice constant of a = 5.62 A, where V = a^3.

For the volume of 177 A3 you get the interatomic distance of ~2.80. This reasonably agrees with the sum of ionic radii 2.20+0.60 A
http://www.webelements.com/
PS In your POSCAR in combination with direct (fraction) atomic coordinates you should use "direct" instead of "cart".