TURTLE program

In the development of the TURTLE program [3], we started by considering a multi-structure Valence Bond wavefunction and added the capability to optimise the orbitals. We tried to avoid putting restrictions on the way the wavefunction is built and to allow great flexibility in the choice of orbitals. For... 

In this early application of the TURTLE program, the ability to restrict the wavefunction is used to ascertain the relative importance of 7t-electron delocalisation and induction to the enhanced acidity of carboxylic acids and enols compared to alcohols. These generic classes of molecules are represented by formic acid, vinyl alcohol and ethanol respectively. 

China, 1999 (b) The VBSCF and BOVB methods are incorporated in the TURTLE program ... 

J. H. van Lenthe, F. Dijkstra, W. A. Havenith, in Valence Bond Theory, D. L. Cooper, Ed., Elsevier, Amsterdam, The Netherlands, 2002, pp. 79-116. TURTLE—A Gradient VBSCF Program Theory and Studies of Aromaticity. 

The first output for H2 was calculated with the program TURTLE (4), and is presented in Output 2.1. The relevant output information begins with the section called vb-symbolic , which specifies the multiplicity of the molecule, the number of electrons, and the number of VB configurations (i.e., structures). This process is followed by a symbolic representation of the VB structures, namely, the VB basis set of structures for the problem, or in short, the VB structure-set. Here, the numbers 1 and 2 designate the atomic orbitals (AOs) in which the VB program distributes the bonding electrons. The number 1 is the Is AO of one of the hydrogen atoms, say H1 and 2 is the AO of the other atom, H2. Thus, structure 1, represented as 1 2 , is the covalent (HL) structure, where each atom possesses one electron, while structures 2 and... 

TURTLE used to be freely available as a stand-alone program. But now it is implemented in the GAMESS—UK program (66), which has to be purchased and licensed. 

TURTLE A VB program that can perform a variety of VB calculations (e.g., VBSCF and BOVB). The program is now incorporated into the GAMESS-UK package. 

TURTLE - A gradient VBSCF Program Theory and Studies of Aromaticity... 

The Ab Initio Valence Bond program TURTLE has been under development for about 12 years and is now becoming useful for the non-specialist computational chemist as is exemplified by its incorporation in the GAMESS-UK program. We describe here the principles of the matrix evaluation and orbital optimisation algorithms and the extensions required to use the Valence Bond wavefunctions in analytical (nuclear) gradient calculations. For the applications, the emphasis is on the selective use of restrictions on the orbitals in the Valence Bond wavefunctions, to investigate chemical concepts, in particular resonance in aromatic systems. 

The formalisms derived above were implemented in the Ab Initio Valence Bond program TURTLE [3], The logo for the program is shown in Fig. 1. This is the logo for the parallel version, as is obvious from the number of turtles depicted. 

Our VB program TURTLE [3] allows for both a more extensive and a more restrictive description of benzene and cyclobutadiene than was available in the previous studies. We included full orbital and full geometry optimisation. Two orbital models were used. The first has p-like (pn) orbitals strictly localised on the carbon atoms. This corresponds to the classical Heitler-London model [48], but with optimal orbitals. The second uses delocalised fully optimised [68] p orbitals, which include tails to neighbouring atoms. 

We have given an account of some of the inner workings of the gradient VBSCF program TURTLE. The program is especially conceived to allow the optimisation of wavefunctions of arbitrary form. This feature is exploited in the study of resonance and delocalisation phenomena. 

J. Verbeek, J.H. Langenberg, C.P. Byrman, J.H. van Lenthe, TURTLE-An ab initioVB/VBSCF/VBCI program Theoretical Chemistry Group, Debye Institute, University of Utrecht, 1993. 

Allen and HEART were among the first to publicly assert that voluntary programs would not save the turtles. She began lobbying her congressmen, the congressional committees overseeing endan-... 

TURTLE - An ab initio VB/VBSCF Program, University of Utrecht, Utrecht. 

These turtles were borrowed from the international conservation program which is using "artificial imprinting" to attempt to establish a new nesting beach for this endangered species. Our four-compartment arena was used once again, and the animals were individually evaluated for number of entries, total time spent and time spent per entry into compartments treated with 1) Padre Island seawater used to wash Padre Island sand ... 

VB (BOVB) method, which also utilizes covalent and ionic structures, but in addition allows them to have their own unique set of orbitals. This method is now incorporated into the programs TURTLE and XIAMEN-99. Very recently, Wu et al. developed a VBCI method that is akin to BOVB, but which can be applied to larger systems. The recent biorthogonal VB method (bio-VB) of McDouall has the potential to carry out VB calculations on systems with up to 60 electrons outside the closed shell. And finally, Truhlar and co-workers " ° developed the VB-based multiconfiguration molecular mechanics method (MCMM) to treat dynamical aspects of chemical reactions, while Landis and co-workers " introduced the VAL-BOND method that predicts the structures of transition metal complexes using Pauling s ideas of orbital hybridization. In the section dedicated to VB methods, we mention the main software and methods that we used, and outline their features, capabilities, and limitations. 

The research at Hebrew University is supported in part by a grant (to SS) from the Israeli Science Foundation. The two authors are thankful to all their co-workers during their years of collaboration (1985-present). Joop van Lenthe and Wei Wu are especially thanked for making their programs (TURTLE and XIAMEN99) available to us. Sam de Visser is acknowledged for proofing the chapter. 

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