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

TURTLE - A gradient VBSCF Program Theory and Studies of Aromaticity 

Joop H. van Lenthe, Fokke Dijkstra, Remco W. A. Havenith 

Theoretical Chemistry Group, Debye Institute, Utrecht University, 

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. 

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 

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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. 

Elsevier, Amsterdam, The Netherlands, 2002, pp. 79-116. TURTLE—A Gradient VBSCF Program Theory and Studies of Aromaticity. 

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