Ab-initio SCF calculation

Rinaldi D, M F Ruiz-Lopez and J L Rivail 1983. Ab Initio SCF Calculations on Electrostatically Solvate Molecules Using a Deformable Three Axes Ellipsoidal Cavity. Journal of Chemical Physics 78 834 838. 

You can use any ab initio SCF calculation and all the semi-empiri-cal methods, except Extended Hiickel, for molecular dynamics simulations. The procedures and considerations are similar for simulations using molecular mechanics methods (see Molecular Dynamics on page 69). 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

There is ample evidence [9,17,44] that the INDO SCF procedure transformed according to this scheme (C INDO) can provide predictions comparable to those of minimal-basis-set ab initio SCF calculations for conformations and rotational barriers of conjugated molecules in the ground state. 

Laurence, P. R., and C. Thomson. 1981. A Comparison of the Results of PCILO and Ab Initio SCF Calculations for the Molecules Glycine, Cysteine and N-Acetyl-Glycine. Theoret. Chim. Acta (Berl.) 58, 121-124. 

The heuristic success of the concepts of through-space and through-bond interactions introduced by Hoffmann200 made it desirable to link it to many electron procedures, at least on the level of semiempirical or ab initio SCF calculations. Such a treatment, which had been proposed some time ago204, has been reviewed in detail elsewhere173. For this reason we shall only sketch the essential steps involved, using norbornadiene 75 as an example. 

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. 

Tab. 3.6-4. Relative energies obtained by ab initio SCF calculations of dimeric alkaline-earth metal dihydrides and bis(phosphanides) (kj mol-1).

A speculative proposal was made thirty years ago by Schmid and Krenmayr77, namely that a nitrosyl ion solvated, but not covalently bonded, by a water molecule may be involved in these systems. This hypothesis was investigated theoretically in 1984 by Nguyen and Hegarty78 who carried out ab initio SCF calculations of structure and properties employing the minimal STO-3G basis set, a split-valence basis set plus polarization functions. Optimized geometries of six planar and two nonplanar forms were studied for the nitrosoacidium ion. The lowest minimum of molecular electrostatic potential... 

The first kind of simplification exclusively concerns the size of the basis set used in the linear combination of one center orbitals. Variational principle is still fulfilled by this type of "ab initio SCF calculation, but the number of functions applied is not as large as necessary to come close to the H. F. limit of the total energy. Most calculations of medium-sized structures consisting for example of some hydrogens and a few second row atoms, are characterized by this deficiency. Although these calculations belong to the class of "ab initio" investigations of molecular structure, basis set effects were shown to be important 54> and unfortunately the number of artificial results due to a limited basis is not too small. 

An impressive number of semiempirical and ab initio SCF calculations on molecular associations of different kinds is available now. In many cases very accurate ab initio SCF calculations using extended basis sets were also performed. 

Ab-initio SCF calculations on the water molecule in various model complexes such as (H20)2, Li+H20, and (Li+)2 H20 show a depletion of the density of the lone pair in the internuclear region for long bonds, while for short bonds, such as Li+ O < a 1.6 A and O(H) O < 2.6-2.7 A, the effect is found to be reversed (Hermansson 1985), in accordance with the observations on oxalic acid dihydrate. The induced polarization of the acceptor density towards the Li+ or H atom is apparently still present for the longer distances, but very diffuse and below the lowest contour of most maps (D. Feil, private communication). Exchange repulsion opposing the attractive effect becomes important for larger ions such as K+, for which the oxygen lone pair penetrates the ion s electron... 

Molecular Cl(H)Si=S (126) was also formed in an argon matrix in a photochemically induced reaction of SiS with HC1. From the isotopic splittings (H/D and 35C1/37C1) of the IR absorptions the Cs structure of the species with silicon as the central atom is deduced. By a normal coordinate analysis a value of 4.83 mdynA-1 is obtained for the SiS force constant, a value which was confirmed by ab initio SCF calculations of the IR spectrum51. 

By theoretical prediction of its IR spectrum via ab initio SCF calculation, a structure of Cs symmetry with Si as the central atom was deduced for the photoproduct HClSi=S from Si=S and HC1 in an argon matrix239. 

D. Rinaldi, M. F. Ruiz-Lopez and J. L. Rivail, Ab initio SCF calculations on electrostatically solvated molecules using a deformable three axes ellipsoidal cavity, J. Chem. Phys., 78 (1983) 834-838. 

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