Zero-point corrections, electronic structure

Theoretically transient three-membered rings containing hypervalent silicon as a structural motif (Fig. 1) have been studied at the SCF and DFT level and the relevant bond lengths are shown in Table 1 (substituents at C and E are not included in Table 1). Characterization of the transition state as early or late can be assumed from the difference between C-E and E-Si bond lengths. From Table 2 the method dependence of the barrier to selected 1,2-silyl-shifts can be taken. Generally, inclusion of electron correlation substantially diminishes the barrier while zero-point correction has only a minor effect. Barriers to silyl migration are in general much smaller than those to related... 

The use of computational chemistry to address issues relative to process design was discussed in an article. The need for efficient software for massively parallel architectures was described. Methods to predict the electronic structure of molecules are described for the molecular orbital and density functional theory approaches. Two examples of electronic stracture calculations are given. The first shows that one can now make extremely accurate predictions of the thermochemistry of small molecules if one carefully considers all of the details such as zero-point energies, core-valence corrections, and relativistic corrections. The second example shows how more approximate computational methods, still based on high level electronic structure calculations, can be used to address a complex waste processing problem at a nuclear production facility (Dixon and Feller, 1999). 

Table 5.2 Relative CASSCF(12,11) Zero-Point Energies (ZPEs) and CASSCF(12,11) and CASPT2(12,11) ZPE-Corrected Electronic Energies (kcaPmol) of 14 and Methylnitrene 14 and Methyleneimine 15 and of the Transition Structures Connecting A 14 to 15 (TSl) Computed with cc-pVDZ and cc-pVTZ Basis Sets ...

In electronic structure calculations, the zero point for the energy function has all particles (electrons and nuclei) infinitely removed from each other, and this common reference state allows energies for systems with different numbers of particles to be directly compared. If the same reference is used in force field methods, the energy function becomes an absolute measure of molecular stability. The difference relative to the normal reference state for force field functions is the sum of all bond dissociation energies, at least for a simple diagonal force field. If correction terms are added to the... 

---