Is positive potential energy for pure methanol normal?

GROMACS version:2020.2
GROMACS modification: No

Dear all,

I am learning gromacs and now run an energy-minimization calculation for a pure methanol system. The number of the methanol molecules and the box size are set to match the pure liquid density, namely, 953 molecules in a (4nm)^3 box. The opls-aa force field was used. And the box was generated by packmol.

However, after energy minimization, I got a potential energy of positive value, about 1e4, while the tutorial 1 mentions that the potential energy should be negative (at least fot that lysozyme system).

So I wonder if the positive potential energy normal for a pure methanol system? If not, which part should I check for error analysis?

Many thanks in advance.