CALCULATION OFHO₂DENSITY OF STATES ON THREE POTENTIAL ENERGY SURFACES

Abstract

Density of states (DOS) in both bound and unimolecular dissociation regime for HO 2 system have been calculated quantum mechanically by Lanczos homogeneous filter diagonalization (LHFD) method. Three potential energy surfaces are explored and the results are contrasted for the total angular momentum J = 0 density of states. While two ab initio potential energy surfaces (PESs) (TU PES, J Chem Phys, 115:3621 and XXZLG PES, J Chem Phys 122:244) produce the DOSs which are in fairly good agreement, the semi-empirical double many-body expansion (DMBE) IV PES (J Phys Chem 94:8073) generates the much higher DOSs in higher energy range. The quantum mechanical DOSs are also compared with Troe et al.'s results from harmonic density, semiclassical density and their early density of states on the same TU ab initio surface.

Quantum Dynamical Calculation of Bound Rovibrational States of HO2 up to Largest Possible Total Angular Momentum, J ≤ 130

“…This is with a typical Wyatt window size of around 50 states (a larger window would have led to fewer QMR iterations, but greater computational cost for the preconditioner construction). In contrast, Smith and co-workers required from 10 5 to 10 6 iterations to converge each of their filter diagonalization calculations , although their computational cost per iteration is somewhat less than ours. For so many iterationsarguably the largest ever published at the timereorthogonalization of the Lanczos vectors is certainly not possible, and so those authors must contend with spurious eigenvalues and other headaches.…”

“…The hydroperoxyl radical, HO 2 , has received very substantial experimental − and theoretical − interest over the last two decades. Far from abating over time, this interest seems only to have increased in recent years.…”

“…Of course, a completely rigorous characterization of the validity of the above approximations requires comparison with accurate rovibrational quantum dynamics calculations, as well as a determination of the relevant J range. Although such calculations have been performed for the HO 2 J = 0 states using a variety of different potential energy surfaces (PES’s), ,,,,,,− ,,,,− , comparatively few exact rovibrational bound state calculations have thus far been performed. ,− ,, This is despite much evidence ,,,− ,,− suggesting that HO 2 exhibits substantial Coriolis coupling and otherwise floppy behavior. An important contributing factor here is the extremely large J values that are in practice relevant for this system.…”

“…Note that more recent and accurate PES’s exist, as described in Section III, but have not been used as extensively as DMBE IV in the context of bound rovibrational states. Of particular interest in the most recent HO 2 literature have been studies addressing the density of states (DOS). , The latter work shows marked DOS differences across the different PES’s, particularly at high energies. DMBE IV is found to have the highest DOS of the PES’s consideredimplying that the exact calculations as presented here would be substantially less computationally expensive, if performed using a more recent PES.…”

“…We are not the first to have applied parallel quantum dynamics techniques to the rovibrational HO 2 systemin fact, the history of the former has “paralleled” calculations of the latter since the earliest days that such techniques were developed. − , The main strategy usedin effect, to assign one K block to each core or nodeis by and large that still adopted in more recent studies, such as those from the Smith and Guo groups. − ScalIT , on the other hand, adopts an entirely different approacha much more powerful, flexible, and scalable strategy, focused on parallelization at the matrix-vector product level, and thus allowing for true, distributed-memory, massive parallelization. (See Paper I for additional discussion, both of the history of parallel quantum dynamics calculations, as well as ScalIT itself.)…”