NDDO-SRP

A promising recent development concerns the use of semiempirical NDDO methods with specific reaction parameters (NDDO-SRP) [144-147] in direct dynamics calculations. In these studies the parameters in the standard AMI method are carefully adjusted to optimize the potential surface for an individual reaction or a set of related reactions (typically allowing parameter variations up to 10% from the original values). When adjusting with respect to experimental data, NDDO-SRP is required to reproduce the exothermicity and the barrier (or rate constant) of the reaction investigated. Under these circumstances NDDO-SRP then predicts reasonable transition structures and force fields for the reaction which is consistent with previous experience [48,49]. Direct dynamics calculations on such NDDO-SRP surfaces have provided very encourag-... 

A method that improves the accuracy of a semiempirical PES for a particular reaction is NDDO-SRP of Truhlar and co-workers (NDDO with specific reaction parameters recall that MNDO, AMl.and PM3 are all NDDO methods). Here, some of the parameters in the AMI method are adjusted either to reproduce such experimental data as the reaction s energy change, barrier height, and rate constant or to reproduce a small number of points on the PES calculated with an ab initio method that includes correlation. See the references cited in W. Thiel, Adv. Chem. Phys., 93,731 (1996). 

One way to keep the cost of the calculations low but improve the accuracy is to use semiempirical molecular orbital calculations in which some of the parameters are fit to data for the specific reaction of interest or for a limited range of reactions. We call this approach SRP for specific reaction parameters or specific range parameters. In several applications we have combined the SRP approach with semiempirical molecular orbital theory employing the neglect of diatomic differential overlap (NDDO) approximation. This is called the NDDO-SRP approach [49]. 

If HL denotes the high level (e.g., QCISD(T)/aug-cc-pVTZ or QCIS(T)/aug-cc-pVTZ//MP2/aug-cc-pVTZ) and LL denotes the low level (e.g., NDDO-SRP or MP2/6-31G ), then the final result of these three steps is denoted HL///LL, which is a direct generalization of the // notation of electronic structure theory. In particular //LL means that stationary point geometries are calculated at level LL, whereas ///LL mans that the reaction path is calculated at level LL. 

The Hrst low-level theory is an NDDO-SRP calculation in which the functional form of the PM3 version of NDDO theory was generalized to allow a larger number of resonance integral parameters. In the original AMI and PM3 parameterizations, there are five resonance parameters for a system composed of H, C, and N, namely Phs. Pcs Pcp. Pns> and PNp. Resonance parameters Px/X7 - ° interaction of an -type orbital on atom X with an / -type orbital on atom X are then approximated by... 

Table 6. Arrhenius activation energies from fits to calculated rate constants at pairs of temperatures in the low, middle, and high-temperature regimes, based on NDDO-SRP as the low level and CVT/pOMT dynamics.

There is also work with the RPH based on semiempirical methods. However, semiempirical methods are parametrized for equilibrium geometries and, accordingly, do not necessarily represent all parts of a reaction valley in a consistent way. Because of this, Truhlar and co-workers have proposed modifications of known semiempirical methods so that all their adjustable parameters are varied to reproduce experimental or ab initio data for specific reactions. Since most of the semiempirical methods presently in use are based on the NDDO approach, the term specific reaction parameter NDDO model (NDDO-SRP) has been coined." The SRP parametrization changes the NDDO model from being qualitatively incorrect to semiquantitatively accurate and, accordingly, provides a much cheaper basis to apply the RPH. 

Both the dual-level descriptions and the NDDO-SRP approach have to be applied with care since they can lead to discontinuities and spurious minima in the PES function. Also, there are cases for which the reparametrization of a NDDO method is too difficult to provide more than just a qualitative insight into the reaction dynamics. Clearly, dual-level description and NDDO-SRP methods are only of limited value for a detailed investigation of reaction mechanism, energy transfer, and energy dissipation with the help of the RPH. 

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