A combination of quantum mechanics and molecular mechanics QM/MM has been used to study the capture of ethylene by Brookhart's Ni diimine catalysts of the type (ArN=C(R')-C(R')=NAr)Ni-II-propyl(+) with (1) R' = H and Ar = H, (2) R' = H and Ar = 2,6-C6H3(i-Pr)(2), or (3) R' = CH3 and Ar = 2,6-C6H3(i-Pr)(2) The study made use of both conventional "static" density functional theory (DFT) based calculations as well as slow growth first principle molecular dynamics (FPMD) DFT methods to examine the capture of ethylene. Examination of the static potential energy surface of all. three catalyst models 1, 2, and 3 reveals that there is no enthalpic barrier to the capture process. However, both the static and molecular dynamics simulations suggest that there is an entropic barrier to the-association that originates from the loss of rotational and translational entropies upon association. The;FPMD QM/MM slow growth barriers were calculated to be 7.5, 10.3, and 10.8 kcal/mol at 300 K for catalysts 8, 2, and 3 , respectively. An analysis suggests that the trend in the barriers can be related to the size of the; active site. The free energy barrier for the pure QM model of 1 has also been estimated from a series of frequency calculations. This approach provides a barrier of 7.7 kcal/mol (and 6.8 kcal/mol without quantum dynamical contributions), which is in fair agreement with the 7.5 kcal/mol barrier (without quantum dynamical contributions) calculated from the slow growth simulations. Analysis of the estimate from the frequency calculations suggests that this barrier estimate represents an upper limit, since the components of the vibrational entropy that compensate the loss of rotational and translational entropy upon association are partially neglected in the treatment.