Oxygen atoms in SDS getting too close

GROMACS version: 4.0.5
GROMACS modification: Yes/No
Here post your question Hello, dear users
I’m performing a simulation with SDS molecules and water. The polar head oxygen atoms of the surfactant, however, get too close inmediately after initiating the simulation, but nothing is wrong with the .itp file, as I have already tested it with isolated SDS molecules. What could be the reason?

Best wishes,
Álvaro