Molecular geometries by the Extended Hueckel Molecular Orbital (EHMO) method

Abstract

Bond length calculations with the extended Hiickel molecular orbital (EHMO) approximation can be improved by adding a two-body repulsive energy term and a distance-dependent Wolfsberg-Helmholz constant K = k exp (SR). As demonstrated in the case of simple homonuclear two-level interactions, such a distance-dependent K leads to problems, however. This drawback of the otherwise very useful improvement of the EHMO method can be overcome by applying = 1 + exp (-5(7?

Chemical reaction dynamics of PeCB and TCDD decomposition: A tight‐binding quantum chemical molecular dynamics study with first‐principles parameterization

“…On the basis of the improvement in the Wolfsberg-Helmholz formula by Anderson [20] and Calzafferi et al [7], we used the following expression for the nonorthogonal term of the Hamiltonian, H rs :…”

“…It is well known that 3,3Ј,4,4Ј,5-penta-chlorinated biphenyl (3,3Ј,4,4Ј,5-PeCB) and 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (2,3,7, are the most toxic compounds among the PCBs and PCDDs, respectively. They are emitted into the environment mostly from incomplete combustion or incineration of organic wastes containing halogens [1].…”

“…In this regard, Laasonen and Nieminen [6] proposed a tight-binding quantum chemical molecular dynamics (TB-QCMD) method based on the TB approximations, which is much faster than the CarParrinello method. The TB approach has been significantly modified and advanced from the original formula by several improvements, such as adding a two-body repulsive energy term [7], a distancedependent Wolfsberg-Helmholz constant or the atom superposition and electron delocalization [8], the self-consistent charge [9], the self-consistent charge and self-consistent configuration [10], and ab initio parameterization [11][12][13]. We integrated these improvements for the TB approximations into the TB-QCMD method and have succeeded in developing a novel TB-QCMD program, Colors.…”

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ABSTRACT:The decomposition reaction dynamics of 2,3,4,4Ј,5-penta-chlorinated biphenyl (2,3,4,4Ј,3,3Ј,4,4Ј,3Ј,4,4Ј,and 2,3,7,3,7, was clarified for the first time at atomic and electronic levels, using our novel tight-binding quantum chemical molecular dynamics method with first-principles parameterization. The calculation speed of our new method is over 5000 times faster than that of the conventional firstprinciples molecular dynamics method.

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Performance of the semiempirical PM3 (tm) method in the geometry optimization of transition metal complexes

“…Some attempts have, nevertheless, been made to introduce d-orbitals in the traditional semiempirical methods, and one can mention MNDO/d, 11,12 SAM/1, 13,14 PM3-d, 15 ZINDO, 16,17 and EHMO-ASED. 18 None of these methods has, however, reached a widespread use, maybe because of poor distribution of the codes. Things may be currently changing with the appearance of the PM3 (tm) method.…”