Artur Michalaka,b and Tom Zieglera,* aDepartment of Chemistry, University of Calgary, University Drive 2500, Calgary, Alberta, Canada T2N 1N4
bDepartment of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, R. Ingardena 3, 30-060 Cracow, Poland

Presented is an algorithm for performing ab initio molecular dynamic (MD) simulations along a pre-determined intrinsic reaction path (IRP). The proposed approach has been implemented within the projector-augmented-wave DFT methodology (PAW program). The slow-growth MD simulations along the IRP has been applied to : (i) the HCN Æ CNH isomerization reaction, (ii) the conrotatory ring opening of cyclobutene, (iii) the prototype SN2 reaction: Cl- + CH3Cl  Æ  ClCH3 + Cl-,  (iv) the chloropropene isomerization: Cl-CH2-CH=CH2  Æ  CH2=CH-CH2Cl. The results demonstrate that the slow-growth MD approach along a pre-determined IRP leads to smooth free-energy profiles; use of a well-defined reaction coordinate (RC) reduces the problem of the free-energy hysteresis. Thus, the slow-growth simulations along the IRP typically require less timesteps than the standard approach with an a priori chosen RC. Illustrative examples show that the applied methodology works well for reactions involving concerted changes in many geometrical variables as well as for  cases where the finite-temperature path strongly deviate from the IRP.