Turbomole methods

The CC2 method [74] is an approximation to coupled cluster with singles and doubles (CCSD), and the excited state energies calculated have MP2 quality. An implementation that employs the resolution of identity (RI) approximation for two-electron integrals to reduce the CPU time is also available, RI-CC2 [75], which is suitable for large scale integral-direct calculations. This method has been implemented in TURBOMOLE [76],... 

Ahlrichs R, von Arnim M (1995) TURBOMOLE, parallel implementation of SCF, density functional, and chemical shift modules. In dementi E, Corongiu G (eds) Methods and techniques in computational chemistry. STEF, Cagliary Eichkorn K, Treutler O, Ohm H, Haser M, Ahlrichs R (1995) Chem Phys Lett 242 652 Becke AD (1988) Phys Rev A 38 3098 Perdew JP (1986) Phys Rev B 33 8822 Garrou PE (1985) Chem Rev 85 171 and references cited therein... 

All of the systems were initially optimized using a much higher level of theory, in order to ensure that the OM2 method provides a realistic description of the structure. The method employed was the second-order Mpller-Plesset perturbation theory (MP2) [50] using the cc-pVDZ basis set [51]. The resolution-of-identity (RI) approximation for the evaluation of the electron-repulsion integrals implemented in Turbomole was utilized [52]. 

R. Ahlrichs, M. Bar, M. Haser, H. Horn, C. Koknel, Electronic structure calculations on workstation computers The program system Turbomole, Chem. Phys. Lett. 162 (1989) 165 M. Haser, R. Ahlrichs, Improvements on the direct SCF method, J. Comput. Chem. 10 (1989) 104 O. Treutler, R. Ahlrichs, J. Chem. Phys. 102 (1995) 346 R. Bauernschmitt, R. Ahlrichs, Treatment of Electronic Excitations within the Adiabatic Approximation of Time Dependent Density Functional Theory, Chem. Phys. Lett. 256 (1996) 454 S. Grimme, F. Furche, R. Ahlrichs, An improved method for density functional calculations of the frequency-dependent optical rotation, Chem. Phys. Lett. 361 (2002) 321 F. Furche,... 

Implementations of the spin-free DKH Hamiltonian exist by now for many standard quantum chemistry packages like Molecule-Sweden, Columbus, Turbomole, Molcas and Nwchem. The method has also been implemented in several programs for the calculation of periodic structures, in particular crystals (Boettger 1998b Fehrenbach and Schmidt 1997 Geipel and Hess 1997). 

The stnictures of the isomeric zwitterions 6a and 6b and five different structures of the anion 7 were studied by quantum-chemical methods. For this purpose, geometry optimizations at the SCF level with an optimized SVP basis set [23] were performed using the TURBOMOLE program system [24], Generally, the results of the computational studies were in good agreement with the experimentally established data. 

The main goal of the present work is to report the implementation of the explicitly-correlated coupled-cluster singles-and-doubles method (CCSD(F12)) in Turbomole. This tool is capable of very efficient calculating the CCSD energies at the basis set limit with relatively small orbital basis sets. The implementation works with RHF, UHF and ROHF reference wave functions, which means that it can treat both closed- and open-shell species. The formulation in terms of intermediate quantities and the application of density fitting techniques make this implementation quite unique. 

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