Graphene Band Calculations

I am using OpenMx 3.8 compiled with option -O2. The test runs showed that the compilation is successful with this flag. I am running a graphene system with 4 carbon atoms in a unit cell. In accordance with this unit cell, the Dirac point lies at P-point in reciprocal space. To do band calculations, if i use an E-cut of 250Ry (which i assume is already quite high) and grid size of (13 13 1) with C6-s2p1 PAO's, i get a band gap of about 0.001ev with cohesive energy 8.1ev. My question is why i am not getting a zero-band gap (-O2 flag was used for compilation) ? Please suggest

This is the input file that i am using for my calculations