Specific reaction parameters, SRP

A specific reaction parameter (SRP) method has been used to reparameterize the semiempirical AMI calculations of the reactions between OH and CH3-F and CH3-CI in the gas phase and in water. The AMI-SRP method predictions are at a much lower cost (faster) than MP2/aug-cc VDZ calculations the AMl-SRP method also predicts an intrinsic reaction coordinate in the gas phase and activation energies in solution that are in good agreement with a MP2/aug-cc-pVDZ QM/MM result... 

Semiempirical direct dynamics was used to study trimethylene s unimo-lecular dynamics and the thermal stereomutation of cyclopropane.The semiempirical model used in these simulations is AMI with specific reaction parameters (SRPs see discussion of semiempirical electronic structure theory in the section on BO direct dynamics) chosen to fit the CASSCF PES. In choosing the SRPs, the AMI barrier for propene formation was lowered by... 

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]. 

LAM = large amplitude motion RP = reaction path RPH = reaction path Hamiltonian RSH = reaction surface Hamiltonian SAM = small amplitude motion SRP = specific reaction parameter SRPH = solution reaction path Hamiltonian. 

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. 

SRP, a term first coined by Rossi and Truhlar (1995), stands for specific reaction (or range) parameters . An SRP model is one where the standard parameters of a semiempirical model are adjusted so as to foster better performance on a particular problem or class of problems. In a sense, the SRP concept represents completion of a full circle in the philosophy of semiempirical modeling. It tacitly recognizes the generally robust character of some underlying semiempirical model, and proceeds from there to optimize that model for a particular system of interest. In application, then, SRP models are similar to the very first semiempirical models, which also tended to be developed on an ad hoc, problem-specific basis. The difference, however, is that the early models typically were developed essentially from scratch, while SRP models may be viewed as perturbations of more general models. 

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