We next examine several active-site models for an enzymatic reaction in which l-aspartate α-decarboxylase (AspDC) catalyzes conversion of l-aspartate to βalanine. This is an essential process in biosynthe- sis of vitamin B5 that produces a precursor to 4phosphopantetheine and coenzyme A in bacteria. Previous modeling by others explored the decarboxylation of l-aspartate using various cluster models of AspDC, focusing on the C-C bond-breaking step leading to liberation of CO 2 . One of these active-site models is depicted in Fig. , derived from the crystal structure of AspDC taken from H. pylori in association with iso-asparagine (PDB: 1UHE). Here, we examine how various anchoratom constraints impact the predicted barrier heights, ZPE, and vibrational entropy as the model size increases.