Diatomics in molecules

By using this approach, it is possible to calculate vibrational state-selected cross-sections from minimal END trajectories obtained with a classical description of the nuclei. We have studied vibrationally excited H2(v) molecules produced in collisions with 30-eV protons [42,43]. The relevant experiments were performed by Toennies et al. [46] with comparisons to theoretical studies using the trajectory surface hopping model [11,47] fTSHM). This system has also stimulated a quantum mechanical study [48] using diatomics-in-molecule (DIM) surfaces [49] and invoicing the infinite-onler sudden approximation (lOSA). 

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... 

A function, which is similar to the London equation and based on the diatomic-in-molecules approach was first used by Kuntz to fit the ab initio surface for collinear reaction... 

A. V. Bochenkova, M. A. Suhm, A. A. Granovsky, and A. V. Nemukhin, Hybrid diatomics in molecules based quantum mechanical/molecular mechanical approach applied to the modeling of structures and spectra of mixed molecular clusters Ar (HCl)m and Ar (HF)m. /. Chem. Phys. 120, 3732 3743 (2004). 

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... 

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). 

Tully, J. C. Diatomics-in-molecules potential energy surfaces. II. Nanadiabatic and spin-orbit interactions, J. Chem.Phys., 59 (1973) 5 122-5134. 

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

Currently, Ellison is using VB theory, diatomics-in-molecules (Pfeiffer et al., 1967) on small species. Companion (1967) has looked into this approach and found that several predictions made in earlier versions... 

The semi-empirical diatomics-in-molecules method370 has been used to study the PE surface of linear HeH, 380 and later it was further used in studies of a variety of first-row hydrides, including linear BeH2.381 BeHa has been rather extensively... 

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]. 

An exceedingly accurate Cl calculation on the surface of H3 recently has appeared [239]. Cl calculations on the avoided intersection of potential energy surfaces for H+ + H2 and H + Hi [240] compared surprisingly well to the earlier diatomics-in-molecules surface of Preston and Tully [241]. Cl calculations were reported for He + H2 B E+, and compared to SCF calculations for He + X 2+ H2 and b H2 [242]. Cl calculations have... 

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. 

Much theoretical work has been carried out on the lithium hydride molecule, which has become the workbench of the theoretical chemist (J ). Browne ( ), and Fraga and Ransil ( 3) have given the binding energy for the LiH ion by ab initio calculation Com-panion(j4) has applied the diatomic-in-molecule theory to the Li H and LiH. molecules and predicted the stabilities of these molecules. We have intensively studied the Li-H system by means of Knudsen effusion mass spectrometry, and identified all predicted molecules and ions as cited above(5), and reported the thermochemical properties of these gaseous species (, 2, ) ... 

---