Ab initio treatments

PIsanI C, DovesI R and RoettI C 988 Hartree-Fock Ab initio Treatment of Crystaiiine Systems (Lecture Notes in Chemistry, voi 48) (Berlin Springer)... 

Equation (165) yields the two components of t(<7, 0), the vectorial non-adiabatic coupling temi, for a distribution of two-state conical intersections expressed in terms of the values of the angular component of each individual non-adiabatic coupling term at the closest vicinity of each conical intersection. These values have to be obtained from ab initio treatments (or from perturbation expansions) however, all that is needed is a set of these values along a single closed circle, each surrounding one conical intersection. 

The traditional valence-only MO schemes are Extended Hiickel and CNDO with its subsequent modifications. Present-day computing facilities make it possible to move one step further, to the ab initio treatment of valence electrons through the use of pseudopotential (PP) methods. The essentials of such methods will be illustrated in the following, through a description of the NOCOR formulation3, which will then be used for extensive calculations on sulphoxide and sulphone systems. The general concepts exposed in the foregoing sections will be illustrated by many examples. 

Acyclic phosphazenes (phosphazo derivatives, phosphine imines, phosphoranimines) continue to attract interest. A review of the three coordinate materials, RN=PR =X has appeared. " Several molecular orbital calculations have been reported. An ab initio treatment of the PN energy surface suggests that this species is best regarded as having a dative phosphorus-nitrogen double bond rather than a triple bond and the phosphonitrene, once formed,... 

That classical calculation may be a density functional theory (DFT) ab initio simulation. An ab initio treatment may be important to handle charge redistribution effects in the condensed phase. 

In this section, we discuss the possibility that the electronic non-adiabatic coupling terms will be derived, not by ab initio treatments but, by solving the curl equations for a given set of boundary conditions obtained from ab initio calculations [114,115]. In other words, instead of performing an ab initio calculation at any point in configuration space we suggest solving the relevant differential equations for boundary conditions obtained from a (limited) ab initio calculation [64-74] or perturbation theory [66,67]. 

If it could be shown that ab initio SCF calculations were effective in at least certain connections, they would of course present obvious advantages in that they are based on a rigorous solution of a specific mathematical problem and so involve no parameters. Consequently they can be applied equally well to systems of all kinds, containing any elements. Semiempirical treatments are limited to systems for which parameters have been determined. Even if computation time presents an inseparable barrier to ab initio treatments of systems large enough to be of chemical interest, such calculations for simpler systems might prove useful as an aid in developing semiempirical treatments. 

The knowledge of the excited state contributions to a and P is of importance not only for understanding the origin of the responses but also for computational purposes. Indeed, for medium-size molecules like / -nitroaniline or small molecules studied with extended basis sets, the full configuration space becomes rapidly very huge and out of reach for a complete ab initio treatment. Therefore, if one needs to tmncate the configuration space, one has to ensure that the most contributing excited states are present and correctly reproduced. 

This is, of course, the approach used by all of the early VB workers. In more recent times, after computing machinery allowed ab initio treatments, this is the sort of wave function proposed by Balint-Kurti and Karplus[34], which they called a multi structure approach. The present author and his students have proposed the multiconfiguration valence bond (MCVB) approach, which differs from the Balint-Kurti-Karplus wave function principally in the way the , are chosen. 

The many successful applications of nitrile oxide cycloadditions in synthesis are intimately linked with theory, both the simple FMO variety as well as the more sophisticated ab initio treatment, where the work of Sustmann and subsequently of Houk and his group has been seminal. We, the practitioners, have thus been supplied with a consistent view on the nature of 1,3-dipoles, their reactivity toward dipolarophiles, and the origin and interpretation of stereoselectivity of cycloaddition chemistry. It is of course desirable that our understanding of the relative reactivities of alkenes as well as of many 1,3-dipoles would be also improved, thereby leading to simple, extended recipes for the chemist practicing synthetics. We hope that this account will stimulate further advances in this field of cycloaddition chemistry and promote further uses of nitrile oxides in organic synthesis. 

The wavepacket calculation for the femtosecond pump-probe experiment presented in Fig. 16 (bottom) is the result of the first consistent ab initio treatment for three coupled potential-energy surfaces in the complete three-dimensional vibrational space of the Naa molecule. In order to simulate the experimental femtosecond ion signal, the experimental pulse parameters were used duration A/fWhm = 120 fs, intensity I - 520 MW/cm2, and central... 

Vol. 48 C. Pisani, R. Dovesi, C. Roetti, Hartree-Fock Ab Initio Treatment of Crystalline Systems. V, 193 pages. 1988. 

Engel, V., Schinke, R., and Staemmler, V. (1988). Photodissociation dynamics of H2O and D2O in the first absorption band A complete ab initio treatment, J. Chem. Phys. 88, 129-148. 

My other hope is that... a basically new ab initio treatment capable of giving chemically accurate results a priori, is achieved soon. 

The two-electron integrals (Equation 6.32) are determined from atomic experimental data in the one-center case, and are evaluated from a semiempirical multipole model in the two-center case that ensures correct classical behavior at large distances and convergence to the correct one-center limit. Interestingly, this parameterization results in damped effective electron-electron interactions at small and intermediate distances, which reflects a (however less regular) implicit partial inclusion of electron correlation (Thiel, 1998). In this respect, semiempirical methods go beyond the HF level, and may accordingly be superior to HF ab initio treatments for certain properties that have a direct or indirect connection to the parameterization procedure. 

Dannenberg, J.J., Hydrogen bonds a comparison of semiempirical and ab initio treatments, J. Mol. Struct. (Theochem), 401, 279-286, 1997. 

The important question of the stability of the perturbation expansion to changes in hybridization and the bond polarity parameter is discussed by Jordan et a/.152 The method has been extended to radicals with a well localized odd electron153 and to some excited states.154 A similar ab initio treatment has been developed but can only be applied to small molecules.155 In a paper dealing with calculation of n.m.r. coupling constants, Dennis and Malrieu make some use of INDO terms rather than CNDO.156 A complete derivation of the PCILO equations using diagramatic techniques has been given recently by Kvasnicka.157... 

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