Wave function of Coulson and Fischer

On the Wave Function of Coulson and Fischer A Third Way in Quantum Chemistry... 

Abstract The wave function of Coulson and Fischer is examined within the context of recent developments in quantum chemistry. It is argued that the Coulson-Fischer ansatz establishes a third way in quantum chemistry, which should not be confused with the traditional molecular orbital and valence bond formalisms. The Coulson-Fischer theory is compared with modern valence bond approaches and also modern multireference correlation methods. Because of the non-orthogonality problem which arises when wave functions are constructed from arbitrary orbital products, the application of the Coulson-Fischer method to larger molecules necessitates the introduction of approximation schemes. It is shown that the use of hierarchical orthogonality restrictions has advantages, combining a picture of molecular electronic structure which is an accord with simple, but nevertheless empirical, ideas and concepts, with a level of computational complexity which renders praetieal applications to larger molecules tractable. An open collaborative virtual environment is proposed to foster further development. 

Fig. 5. Contribution of ionic terms to the binding energy as a function of internuclear distance, (a) Contribution /i of ionic terms for H2 molecule (see equation 25) calculated with best screening constant of a Heitler-London wave function (after Coulson and Fischer (134)). (b) Variation of the binding energy...

The purpose of this essay is to examine the Coulson-Fischer wave function [18] and the approach of Coulson and Fischer from the perspective of contemporary... 

The wave function of H2 is expressed below as a formally covalent VB structure F< > using Coulson—Fischer (CF) orbitals [Pg.71]

Sixty years ago, in 1949, Coulson and Fischer published a seminal paper [18] in the Philosophical Magazine, entitled Notes on the Molecular Orbital Treatment of the Hydrogen Molecule. In this note, they presented a wave function for the hydrogen molecule, which, whilst retaining a simple physical picture, combines the advantages of the two rival theories of molecular electronic structure, mo and VB theories. Let us briefly summarize the discussion given by Coulson and Fischer. 

They consider two forms of an approximate wave function for the ground state of the hydrogen molecule which depend on a single parameter. These wave functions correspond to the vb and mo wave function for a particular choice of the parameter. The first wave function considered by Coulson and Fischer is written as... 

The second form of the approximate wave functions considered by Coulson and Fischer is... 

Wave function (4) is the standard configuration interaction expansion in mo theory. The parameter can take values from -1 to -1-1. Putting /r = 0 gives the pure molecular orbital description first considered by Coulson [49] in 1937. Table 1 summarizes the behaviour of the approximate wave functions as a function of the parameters k and /r. in the Coulson-Fischer analysis. (This table is taken from the work of Coulson and Luz [50].)... 

Coulson and Fischer demonstrated that the two forms of the approximate wave function, (1) and (4), are equivalent if... 

One inconvenience of using the expression vB-fuii (Eq- [-5]) is its relative complexity compared to the simpler HL function (Eq. [3]). Coulson and Fischer proposed an elegant way of combining the simplicity of Fhl with the accuracy of vB-fuii- In the Coulson-Fischer (CF) wave function, I cF, the two-electron bond is described as a formally covalent singlet coupling between two orbitals and (p, which are optimized with freedom to delocalize over the two centers. This is exemplified below for H2 (dropping once again the normalization factors) ... 

In a remarkably undervalued paper, Coulson and Fischer, using the H2 molecule as a simple example, showed that the ionic structures express nothing more than the deformation of the atomic orbitals that occurs when they participate in chemical bonds. They showed that wave function (3) can be rewritten as... 

The results of a valence bond treatment of the rotational barrier in ethane lie between the extremes of the NBO and EDA analyses and seem to reconcile this dispute by suggesting that both Pauli repulsion and hyperconjugation are important. This is probably closest to the truth (remember that Pauli repulsion dominates in the higher alkanes) but the VB approach is still imperfect and also is mostly a very powerful expert method [43]. VB methods construct the total wave function from linear combinations of covalent resonance and an array of ionic structures as the covalent structure is typically much lower in energy, the ionic contributions are included by using highly delocalised (and polarisable) so-called Coulson-Fischer orbitals. Needless to say, this is not error free and the brief description of this rather old but valuable approach indicates the expert nature of this type of analysis. 

Here, a and b are purely localized AOs, while Coulson—Fischer orbitals energy minimization, are generally not very delocalized (e < 1), and as such they can be viewed as distorted orbitals that remain atomic-like in nature. However, minor as this may look, the slight delocalization renders the Coulson—Fischer wave function equivalent to the VB-full wave function (Eq. 3.4a) with the three classical structures. A straightforward expansion of the Coulson—Fischer wave function leads to the linear combination of the classical structures in Equation 3.6. 

Since c1 and c2 are variationally optimized, expansion of v1,mo-ci should lead to exactly the same VB function as vB-fuii in Equation 3.4, leading to the equalities expressed in Equation 3.62 and to the equivalence of mo-ci and vB-fuii (see Exercise 3.1) The equivalence also includes the Coulson Fischer wave function Th h (Eq. 3.5) which, as we have seen, is equivalent to the VB-full description (see Exercise 3.2). 

All the VB methods that deal with semilocalized orbitals use a generalization of the Coulson—Fischer idea (12), whereby a bond is described as a singlet coupling between two electrons in nonorthogonal orbitals that possess small delocalization tails resulting from the variational orbital optimization. Albeit formally covalent, this description implicitly involves some optimal contributions of ionic terms, as a decomposition of the wave function in terms of pure AO determinants would show (see Eqs. 3.5 and 3.6). For a polyatomic... 

An important feature of the BOVB method is that the active orbitals are chosen to be strictly localized on a single atom or fragment, without any delocalization tails. If this were not the case, a so-called "covalent" structure, defined with more or less delocalized orbitals like, e.g., Coulson-Fischer orbitals, would implicitly contain some ionic contributions, which would make the interpretation of the wave function questionable [27]. The use of pure AOs is therefore a way to ensure an unambiguous correspondence between the concept of Lewis structural scheme and its mathematical formulation. Another reason for the choice of local orbitals is that the breathing orbital effect is... 

The Coulson-Fischer wave function for H2 can be considered as the start of the Unrestricted Hartree-Fock (UHF) approach in quantum chemistry, which is the most general single determinant method. We shall not proceed further along this line, but instead ask ourselves if there is a way to correct the simation such that we obtain a wave function that dissociates correctly while preserving the spin and space symmetry of the wave function. The CF wave function gives acmally a hint. What happens if we simply skip the trouble-some triplet term in Eq. (22). This gives rise to a wave function that is a linear combination of two closed shell determinants ... 

Hence the approximation (1), which is a prototype of multistructure valence bond theory, is equivalent to approximation (4), a prototype of molecular orbital configuration interaction theory, and both (1) and (4) are equivalent to (13), the Coulson-Fischer wave function. 

Evidence for this renaissance is seen in the number of monographs [53-55], edited volumes [56-58] and review articles [52, 59, 60, 62, 63, 61, 64-69] on valence bond theory published in recent years. These works display a rich variety of theoretical machinery inspired by the valence bond picture of molecular structure. Some of the methodologies - particularly in the so-called modern valence bond theories introduced by Gerratt and Lipscomb [70,71] under the name spin-coupled wave functions and developed by Gerratt [72,73] and his collaborators [68,74-86] over the past 40 years - exploit the Coulson-Fischer ansatz. As we have seen in Section 3 and will consider further in Section 6, the Coulson-Fischer theory presents a third way of constructing approximate molecular wave functions which combine many of the advantages of both molecular orbital theory and valence bond theory. 

In a paper pubhshed in 1953 as part of a series under the general title The molecular orbital theory of chemical valency, Hurley, Lennard-Jones and Pople [87] presented A theory of paired electrons in polyatomic molecules. The pah-function model of Hurley et al. employed a Coulson-Fischer-type wave function to describe each pair of electrons in a polyatomic molecule. Orthogonality constraints were imposed between orbitals associated with different pairs of electrons in order to render the theory practical, i.e. computationally tractable. Hurley presented the corresponding orbital equations in a subsequent paper [88] which was published in 1956. 

---