Electronic and magnetic properties of an organic multiferroic

C2H5NH3)2CuCl4 was proposed to be a ferroelectric material with dominant ferromagnetic interactions. Using first-principles calculations we find that the insulating state and in-plane ferromagnetism induced by dz2 � x2 � dz2� y2 orbital order can be expressed well by generalized gradient approximation (GGA) at ambient pressure. While the Jahn Teller distortion (JTD) is suppressed under critical pressure of 4 GPa, as a result the new dz2 orbital order leads to in-plane antiferromagnetism. Meanwhile the conventional GGA gives a wrong metal state. Our results suggest that (C2H5NH3)2CuCl4 is a strong correlated charge transfer insulator, while the depress of JTD by pressure changes it to a Mott Hubbard insulator.