Structure is distored after NVT dynamics

Hi everyone
I construct a phenanthrene molecule and opimized its structrue.Then I heat it to 1600k with NVT dynamics,but the structure is distored(the carbon atoms are not at the same plane),then I use NVE dynamics to relaxed,but the strucuture is still distored.But as far as I know the carbon atoms should be at the same plane.What is wrong with my calculation.The forcefiled is PCFF,the thermostat is Berendsen with decay constant 0.1.

Sincerely
Li