Advanced ab initio methods

Three types of surface are in use for water simulations. The first consists of simple empirical models based on the LJ-C potential. There seems to be no purpose in continuing to develop and use such models as they give little, if any, new information. A second group attempts to improve the accuracy of the potential using semiempirical methods based on a comprehensive set of experimental data. These models allow for physical phenomena such as intramolecular relaxation, electrostatic induced terms, and many-body interactions, all of which are difficult to incorporate correctly in liquid water theories. There is room for much more work in these areas. The third group makes use of the most advanced ab initio methods to develop accurate potentials from first principles. Such calculations are now converging with parameterized surfaces based on accurate semiempirical models. Over the next few years it seems very likely that the continued application of the second and third approaches will result in a potential energy surface that achieves quantitative accuracy for water in the condensed phase. 

DFT computations can be extended to considerably larger molecules than advanced ab initio methods and are being used extensively in the prediction and calculation of molecular properties. A recent study, for example, examined the energy required for ionization of very strong acids in the gas phase. Good correlations with experimental values were observed and predictions were made for several cases that have not been measured experimentally, as shown in Table 1.14. 

Advanced ab initio Methods, Density Functional Theory and Solid-state Quantum Mechanics... 

DFT) methods have been recently evolved as an important complement to advanced ab initio methods. DFT may be one of the few applicable calculation methods for large molecule systems and have frequently been used. In general their overall accuracy is lower than that achieved with high-level ab initio schemes. The accuracy of the Density Functional Theory can, however, be improved by use of isodesmic reactions. Most of the calculations are based on isodesmic reactions, in which calculated values are combined with experimental or computed enthalpies of formation of suitable reference systems. The use of isodesmic reaction implies that the number of bonds of each formal type is conserved. Here, another problem arises, namely the experimental enthalpies of formation of the reference species needed in isodesmic reactions are not always known or the values are affected by a large error. 

We have completed two complementary gas/surface collision studies with squalane, using incident 0( P) with 5 and 1 eV and F( P) with 1 and 0.5 eV initial translational energy [28,29]. To understand the viability of using MSINDO for the QM method, we computed the reaction enthalpies and barriers for the H abstraction and elimination reactions of O/F with methane and ethane, and compared the results to those from the advanced ab initio method CCSD(T). These comparisons show that for the reaction barriers, which are important for the bond making/breaking events in our simulations, the MSINDO computations are comparable ( 0.15 eV) to the CCSD(T) results. For the methane reaction, the H abstraction reaction (O + CH4 —> CH3 + OH) has an A rxn = —0.342 eV whereas with F the reaction is more exothermic (as expected) with an AE n = 1.418 eV, based on MSINDO. 

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