Strength of effective Coulomb interaction in two-dimensional transition-metal halides <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>M</mml:mi><mml:msub><mml:mi>X</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>M</mml:mi><mml:msub><mml:mi>X</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>M…

We calculate the strength of effective onsite Coulomb interaction (Hubbard $U$) in two-dimensional (2D) transition-metal (TM) dihalides MX$_2$ and trihalides MX$_3$ (M=Ti, V, Cr, Mn, Fe, Co, Ni; X=Cl, Br, I) from first principles using constrained random-phase approximation. The correlated subspaces are formed $t_{2g}$ or $e_g$ bands at Fermi energy. Elimination efficient screening taking place these narrow gives rise to sizable parameters U between localized ($e_g$) electrons. Due this large interaction, we find $U/W >1$ (with band width $W$) most TM halides, making them strongly materials. Among metallic halides paramagnetic state, correlation $U/W$ reaches a maximum NiX$_2$ CrX$_3$ with values much larger than corresponding elementary TMs other compounds. Based on Stoner model calculated $U$ $J$ values, discuss tendency electron spins order ferromagnetically.