Ab-initio molecular orbital codes

Tel. 904-392-1597, fax. 904-392-8722, e-mail aces2 qtp.ufl.edu Ab initio molecular orbital code specializing in the evaluation of the correlation energy using many-body perturbation theory and coupled-cluster theory. Analytic gradients of the energy available at MBPT(2), MBPT(3), MBPT(4), and CC levels for restricted and unrestricted Hartree-Fock reference functions. MBPT(2) and CC gradients. Also available for ROHE reference functions. UNIX workstations. 

Tel. 904-392-1597, fax 904-392-8722, e-mail aces2 qtp.ufl.edu Ab initio molecular orbital code specializing in the evaluation of the correlation energy using many-body perturbation theory and coupled-cluster theory. Ana-... 

Theoretical studies have been based on ab-initio molecular orbital calculations performed using either the Gaussian 86 code or, earlier, Gaussian codes on a Cray 1 Computer. The majority of calculations employed a modestly sized basis set (6-31G)to save computational time but a few have been carried out using a larger (6-311G ) basis set and using Moller-Plesset (MP) many-body perturbation theory to second or fourth order. 

The PIXEL code and documentation (OPiX package) can be downloaded at the author s site http //users.unimi.it/gavezzot. Computing times on an ordinary PC are of the order of a few seconds for a molecular dimer, to a few minutes for a complete crystal structure. PIXEL has been repeatedly shown to give results of accuracy comparable to ab initio MO methods [51]. For an example. Fig. 5 [52] shows a comparison between MP2, PIXEL, uncorrected and corrected periodic-orbital... 

The detailed study of electron distribution rearrangements in a chemical reaction needs a multiconfiguration ab initio method, because one determinantal wavefunctions are unreliable away from equilibrium geometries. By means of the CASSCF method it is possible to focus attention on the electrons more directly involved in the reaction, allowing the calculation to be done with a relatively limited number of Slater determinants. Moreover, CASSCF uses orthogonal orbitals which are simpler than non-orthogonal orbitals in the development of computer codes. Nowadays CASSCF is, in fact, efficiently included in practically all distributed packages for molecular quantum calculations. 

Approximate molecular orbital theory continues to play an important role, especially as an adjunct to experimental work. However, by the late 1960s, as computers became faster and computer codes improved, theoreticians began applying ab initio methods to metal complexes [24]. 

Rehr and Albers (2000) reported theoretical approaches to the assignment of XAFS spectroscopy. They described the advantages and the remaining problems of the multiplescattering (MS) method for the analyses. Multiple-scattering (MS) formalism and first-principle molecular orbital (MO) calculation techniques so far have been proposed for the theoretical analysis of XANES. The ab initio XAFS codes FEFF series program is widely used for this purpose. The detailed information of the FEFF code is available in the Web site of the FEFF and in several references (e.g., Ankudinov et al. 1998 FEFF 2009). 

The Slater-type orbitals were the first to be used in the molecular quantum chemistry semiempirical calculations. Unfortunately, such functions are not suitable for fast calculations of multicenter integrals in ab-initio calculations. Gaussian-type functions (GTFs) were introduced to remedy the difficulties. GTFs are used in basis sets in practically all modern codes for LCAO calculations of molecules. We know two exclusions... 

Tsuchida and Tsukada have developed an adaptive FE algorithm for large-scale electronic structure calculations. " " They utilize a three-dimensional cubic Hermite basis to represent the electron orbitals and have extended their method to Hnear-scaling complexity.MG techniques were employed in the solution of the Poisson problem, and accurate forces suitable for molecular dynamics simulations were computed. The method was tested on diamond and cubic BN lattices and the C o molecule. The FE code has also been applied to simulate Hquid formamide at the ab initio level.Those simulations were used to assess the accmacy of previous simulations using empirical force fields. The current version of the Tsuchida-Tsukada code is called FEMTECK this code incorporates their recently developed linear-scaling technology. 

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