Diatomics in molecules method

Statistical theories, such as those just described, are currently the only practical approach for many ion-neutral reactions because the fine details of the collision process are unknown all the information concerning the dynamics of collision processes is, in principle, contained in the pertinent potential-energy surfaces. Although a number of theoretical groups are engaged in accurate ab initio calculations of potential surfaces (J. J. Kaufman, M. Krauss, R. N. Porter, H. F. Schaefer, I. Shavitt, A. C. Wahl, and others), this is an expensive and tedious task, and various approximate methods are also being applied. Some of these methods are listed in Table VI, for example, the diatomics-in-molecules method (DIM). 

A. The Diatomics-in-Molecules Method, and Mixed Orbital-Valence Bond Methods... 

DIM diatomics in molecules (method) digital imaging microscopy... 

The problem of how to account for different levels of accuracy in calculations at different points on the potential energy surface goes back a long way and there have been many partial solutions. The atom-in-molecules method of Moffitt [9], the diatomics-in-molecules method of Ellison [10], and the counteipoise model of Boys [11] are all successful in some respects. Unfortunately none of them are helpful for the most commonly met problem by those who are ti ng to fit potential functions to data, which is how to join up data produced by different methods on different regions of the surface. I believe there is no answer to this except scientific judgement... 

DIRECT USE OF THE DIATOMICS-IN-MOLECULES METHOD IN LARGE SYSTEMS AND IN DYNAMICAL CALCULATIONS... 

M, B. Faist and J. T. Muckerman, On the valence bond diatomics-in-molecules method. I. A projection operator reformulation,... 

I. Last, Limitations of the diatomics-in-molecules method, Chem, Phys., in press. 

The most commonly used semiempirical for describing PES s is the diatomics-in-molecules (DIM) method. This method uses a Hamiltonian with parameters for describing atomic and diatomic fragments within a molecule. The functional form, which is covered in detail by Tully, allows it to be parameterized from either ah initio calculations or spectroscopic results. The parameters must be fitted carefully in order for the method to give a reasonable description of the entire PES. Most cases where DIM yielded completely unreasonable results can be attributed to a poor fitting of parameters. Other semiempirical methods for describing the PES, which are discussed in the reviews below, are LEPS, hyperbolic map functions, the method of Agmon and Levine, and the mole-cules-in-molecules (MIM) method. 

DIM (diatomics-in-molecules) a semiempirical method used for representing potential energy surfaces... 

Abstract. We present a quantum-classieal determination of stable isomers of Na Arii clusters with an electronically excited sodium atom in 3p P states. The excited states of Na perturbed by the argon atoms are obtained as the eigenfunctions of a single-electron operator describing the electron in the field of a Na Arn core, the Na and Ar atoms being substituted by pseudo-potentials. These pseudo-potentials include core-polarization operators to account for polarization and correlation of the inert part with the excited electron . The geometry optimization of the excited states is carried out via the basin-hopping method of Wales et al. The present study confirms the trend for small Na Arn clusters in 3p states to form planar structures, as proposed earlier by Tutein and Mayne within the framework of a first order perturbation theory on a "Diatomics in Molecules" type model. 

The London equation has in addition been the progenitor of semi-empirical (or semi-theoretical) valence-bond methods of which Moffitt s method of atoms in molecules (95) and Ellison s method of diatomics in molecules(96), the latter not in fact being a direct generalisation of the former, are the most important. It is beyond the scope of this article to give the details of these methods and their modifications and the reader is referred to other reviews that encompass them (85, 97). It is however important to emphasize that they work by using relatively simple polyatomic wavefunctions and introducing corrections to the resulting Hamiltonian matrix... 

DIM Diatomics in molecules. A semiempricial method used to construct potential energy surfaces of polyatomic molecules from the energy of the diatomic fragments. 

A promising semi-rigorous molecular orbital method, PRDDO, partial retention of diatomic differential overlap has been reported in the interim [186]. Also, the method of diatomics-in-molecules has been revived and the derivations extended to include p orbitals appropriately [187]. 

Kuntz et al [60] using the diatomics-in-molecule (DIM) method constructed three slightly dfferent DIM PESs for the two lowest states. All these DIM PESs exhibit no barrier to reaction (insertion or abstraction) on the lowest surface. 

The electronic structures of Group lA and IB metal clusters have been determined using two theoretical methods ah initio molecular orbital theory and the semi-empirical diatomics-in-molecules (DIM)... 

A wide range of theoretical methods has been applied to the study of the structure of small metal clusters. The extremes are represented on the one hand by semi-empirical molecular orbital (Extended Huckel) (8 ) and valence bond methods (Diatomics-In-Molecules) ( ) and on the other hand by rigorous initio calculations with large basis sets and extensive configuration interaction (Cl) (10). A number of approaches lying between these two extremes have been employed Including the X-a method (11), approximate molecular orbital methods such as CNDO (12) and PRDDO (13) and Hartree-Fock initio molecular orbital theory with moderate Cl. 

The interaction between an open shell atom with a closed shell atom or molecule can be modeled with the Diatomics-in-Molecule (DIM) method [11]. This is an approximate approach, however, with a sounel r hemir al mothntion. The total interaction energy between an open shell atom anel all the remaining closed shell atoms in the cduster can be expressed in a pair additive way as Vo/m = X) =i n where r is the raelius vector between the i-th rare gas atom anel the p-atom (open shell atom) anel is the interaction energy between a single rare gas atom... 

Diatomics-in-Molecules (DIM) Method Semi-Empirical Valence-Bond Methods Approximate Pseudo-Potential Theories... 

The trajectory surface hopping method is an additional extension of the classical trajectory method. Potential energy surfaces are constructed for each electronic state involved in the collision. In addition, a function has to be obtained that defines the locus of points at which hops between surfaces can occur. Still another function is necessary which gives the probability of such jumps as a function of nuclear positions and velocities.28 Diatomics-in-molecules surfaces approximated the two lowest singlet potential surfaces of H3. The surfaces have been shown30 to be in good agreement with accurate ab initio calculations by Conroy.31... 

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