The influence of the ligand structure of zirconocene polymerization catalysts on the accessibility of the active reaction center was studied by an ab initio Hartree-Fock method. The variations in the accessibility were elucidated by comparing molecular structures and relative stabilities of 54 bridged zirconocene catalysts with 19 different bridging units, 18 ancillary Cp' ligands, and 18 ligand substituents. Ligand variations gave rise to various steric and electronic effects affecting both the nature and the concentration of active metal centers in the system, such as steric blocking or shielding of the metal center, and stabilization or destabilization of the active cationic species. Comparisons to experimental work demonstrated clear correlations between accessibility of the active reaction center and observed polymerization activity. Furthermore, interactions between the Lewis acidic aluminum centers and Lewis basic functionalities were observed. The consequences of such interactions are discussed.