Optimized valence configuration

The Hartree-Fock method is modified by mixing some important valence electron configurations with the ground-state one 20>. This is called the OVC optimized valence configurations) method. 

The Cl procedure just described uses a fixed set of orbitals in the functions An alternative approach is to vary the forms of the MOs in each determinantal function O, in (1.300), in addition to varying the coefficients c,. One uses an iterative process (which resembles the Hartree-Fock procedure) to find the optimum orbitals in the Cl determinants. This form of Cl is called the multiconfiguration SCF (MCSCF) method. Because the orbitals are optimized, the MCSCF method requires far fewer configurations than ordinary Cl to get an accurate wave function. A particular form of the MCSCF approach developed for calculations on diatomic molecules is the optimized valence configuration (OVC) method. 

Neglect of Diatomic Differential Overlap Natural Spin Orbital Optimized Double Configuration Optimized Valence Configuration Potential Energy... 

A. C. Wahl, PJ. Bertoncini, G. Das, T.L. Gilbert, Recent progress beyond the Hartree-Fock method for diatomic molecules The method of optimized valence configurations, Int. J. Quantum Chem. S1 (1967) 123. 

The atomic orbitals were determined (numerically) together with the two co-efficients. Similar MCSCF calculations on atoms and negative ions were simultaneously performed in Kaunas, Lithuania, by A. Jucys [10]. The possibility was actually suggested already in 1934 in the book by I. Frenkel [11]. Further progress was only made with the advent of the computer. A.C. Wahl and G. Das developed the Optimized Valence Configuration (OVC) Approach, which was applied to diatomic and some triatomic molecules [12,13]. 

Curves that go beyond the Flartree-Fock method have been calculated for certain systems, and the list grows monthly. Of special interest is the series on ground [62-64] and excited states [63, 64] of CO, on NaLi and NaLi+ using an extended Flartree-Fock method with optimized double-valence configurations [65], HeLi by a valence-bond method [66], and so on. A quite complete listing of all nonempirical potential-energy curves calculated through 1967 is included in the NBS report [33]. 

Table 12.1 contains the electronic Hessian of an optimized valence CASSCF wave function of the H2 molecule at the experimental bond distance of 1.40oo- The wave function is a linear combination of the lo ) and o ) configurations, the orbitals of which have been variationally optimized in the cc-pVDZ basis ... 

A configuration interaction calculation uses molecular orbitals that have been optimized typically with a Hartree-Fock (FIF) calculation. Generalized valence bond (GVB) and multi-configuration self-consistent field (MCSCF) calculations can also be used as a starting point for a configuration interaction calculation. 

The first four steps in our procedure lead to a provisional Lewis structure that contains the correct bonding framework and the correct number of valence electrons. Although the provisional stmcture is the correct structure in some cases, many other molecules require additional reasoning to reach the optimum Lewis structure. This is because the distribution of electrons in the provisional structure may not be the one that makes the molecule most stable. Step 3 of the procedure places electrons preferentially on outer atoms, ensuring that each outer atom has its full complement of electrons. However, this step does not always give the optimal configuration for the inner atoms. Step 5 of the procedure addresses this need. 

As the next examples show, the provisional stmcture may contain one or more inner atoms with less than octets of valence electrons. These provisional stmctures must be optimized in order to reach the most stable molecular configuration. To optimize the electron distribution about an inner atom, move electrons from adjacent outer atoms to make double or triple bonds until the octet is complete. Examples and illustrate this procedure. 

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