SEMIEMPIRICAL MNDO, AM1, AND PM3 DIRECT DYNAMICS TRAJECTORY STUDIES OF FORMALDEHYDE UNIMOLECULAR DISSOCIATION

Direct dynamics calculations are performed, using the semiempirical ne glect of diatomic differential overlap (NDDO) molecular orbital theory , to explore the level of electronic structure theory required to accu rately describe the product energy partitioning when formaldehyde diss ociates into hydrogen and carbon monoxide. Trajectories are initiated at the saddlepoint and are propagated for the short time needed to for m products, by obtaining the energy and gradient directly from the NDD O theory. The resulting product energy partitioning is compared to ava ilable experimental data and the findings of two previous trajectory s tudies, including one nb initio trajectory study at the HF/6-31G* lev el of theory [Chem. Phys. Lett. 228, 436 (1994)]. The MNDO, AM1, and P M3 semiempirical Hamiltonians are studied, as well as Hamiltonians bas ed on specific reaction parameters (SRP). For the latter, the original PM3 and AM1 parameters are adjusted to reproduce some ab initio poten tial energy surface properties, such as stationary points and part of the reaction path. A series of NDDO-SRP Hamiltonians are chosen by fit ting different features of a HF/6-3lG* potential energy surface. Only qualitative agreement is found between the product energy distributio ns of the NDDO-SRP Hamiltonians and that of the HF/6-31G* Hamiltonian . This result is consistent with the well known difficulty of reproduc ing a HF/6-31G* Hamiltonian with a NDDO-SRP model, since dynamic corr elation is not treated in nb initio SCF, but is incorporated into semi empirical methods. Trajectory results with NDDO-SRP Hamiltonians, whic h reproduce a few experimental and/or high-level ab initio stationary points, are in poor agreement with the experimental product energy par titioning. Reparameterizing the NDDO Hamiltonian is laborious, and onl y a few properties of the potential energy surface can be reproduced a t the same time. This indicates the limitations of the NDDO-SRP approa ch, which might be well suited for locally interpolating ab initio dat a, but not for quantitatively describing global potential energy surfa ces. (C) 1996 American Institute of Physics.